David Soul has brought all his dirty laundry along to the interview: his girlfriend's sister is arriving from the States on Friday, so he is on a mission to the dry cleaners. What with the laundry bags and the cigarette and the tennis ball for his poodle, David's hands are pretty full, so I relieve him of a grubby blue blanket as we walk on through Waterlow Park to Highgate.

This is David's manor: he arrived in the UK from Los Angeles in 1995, lured by a West End theatre project that didn't come to fruition, and has never left. Now he and Alexa, his American actress girlfriend of eight years, seem entrenched in north London. There are the Hampstead dinners with David's great chum Martin Bell, for whom he campaigned against Neil Hamilton at Tatton; there are tickets to Arsenal matches; and, having rented the poet's childhood home on Highgate West Hill, he has even discovered John Betjeman, for Pete's sake.

Watching the biscuit-coloured Czechy haring after the ball and mashing up daffodils makes David roar with laughter: a big, chuckly, warm noise, insulated with smokers' phlegm. 'If you'd told me I'd have a poodle one of these days, I'd have said, not on your life,' he says. 'You know, I'd have thought a poodle didn't go with this big, rangy guy - I always had labradors. Shows how your self-image changes.'

Czechy, who has Eastern European ancestry, or something like that, loses interest in the ball and noses off in the opposite direction. 'Puppy! Fetch!' calls David, but the dog has something else on his mind. There is a pause while David briskly scoops the poop. So we visit the dry cleaners, offloading the blanket and two dozen scatter-cushion covers, and then the corner shop to buy three more tennis balls and a fresh pack of Marlboro Lights, and then it's back to the little cafe in the park. We sit outside, because David wants to smoke, and though it's chilly and damp and he's not wearing any socks, he seems unaware of the cold.

He's a big bear of a man, given to gesticulation, and seems to exist in a shambolic state of rolling chaos: spilling coffee onto his saucer, patting pockets wildly in hope of a lighter, scattering Danish pastry crumbs and fag ash like a snow machine. There are still plans to do a film follow-up to Starsky and Hutch , the TV cop show that turned David and his co-star PaulMichael Glaser into Seventies icons; but the 57-year-old David, whose face now looks like an unmade bed, would be unlikely casting for the glowing, blond Hutch, who was always evangelical about tofu and yoga.

In his latest role, he's playing a past-it cardiologist in the BBC's Holby City , a character who is unable to accept that 'the older you get, the tighter your jeans'. It's not what you might call his finest hour, but David is sanguine: 'I've done 600 hours of television, as an actor and director,but Holby City will open the door to something else. I don't know what that is yet.'

That's the way he has always worked. Instead of a career plan, he has what he calls a 'throw-it-up-against-the-wall-and-see-what-sticks approach to life'. Quite a lot has been thrown against the wall over the years - a wall which many managers and agents must have smacked their heads against in frustration - and not much has stuck, apart from a brief but glorious stint as a pop heartthrob ('Don't Give Up On Us Baby' and 'Silver Lady') in the late Seventies, before the record company went belly-up. He has done time in mini-series and Michael Winner movies, has directed episodes of Miami Vice and Hill Street Blues, and starred in an antipodean tour of Blood Brothers; and yet in showbiz terms, he has never matched the popularity that came with Starsky and Hutch, with its odd blend of plastic plots, shoot-outs and macho sentimentality. Originally, David wanted to play Starsky, rather than the vegetarian one who never got to drive, but once signed up, he made Hutch his own.

It was a big break for someone who had started off as 'The Covered Man', a singer who found bizarre celebrity by performing electric blues in a balaclava. ('Before I took it off, everyone in New York had an opinion about who The Covered Man was. A columnist on the New York Post thought it was Mayor Lindsey, trying to break into showbusiness, or Bob Dylan, trying to make it straight.')

For a while after leaving the show, he disliked talking about it, impatient for the next chapter, but now he seems to have accepted that as far as the public is concerned, he will never escape Hutch, and has opted for a reconciliation. He still catches the show on TV occasionally - it's always being repeated somewhere - and, though he thinks on a technical level it looks fairly amateur, 'a little sophomoric', he's still proud of the fact that it pioneered the buddy detective series. 'Those guys were really there, really friends.'

And then there was the homoerotic subtext. 'Oh, yeah!' David rubs his hands enthusiastically. 'We were called "the two prime-time homos". We thought that was great fun. We knew we'd made it then.'

David has four ex-wives (Mim, Karen, Patti and Julia), so his relationship with his co-star, Paul Michael Glaser, has proved one of the most consistent of his life. Only yesterday, David called him to say happy birthday. The last time he was in the States, he stayed with Paul. I say they must get some funny comments when people recognise them out together, but David says: 'People feel good. It's a kind of affirmation that everything's OK.'

You can see why this might be the case. At the time when it was first broadcast, Starsky and Hutch was accused of gratuitous violence - it was restricted viewing for most of my generation - but now, for all its hookers and pimps and drug barons, it looks extraordinarily balmy and innocent. Much more balmy than its stars' off-screen lives. Poor Paul Michael Glaser lost his wife and child to Aids. In the early Eighties, David was ordered to attend therapy classes for alcoholism after attacking his second wife, Patti, when she was seven months pregnant. At various points, he admits, he really lost the plot.

'If you're working 16, 18 hours a day, television gets turned into something of huge importance. It's just a job, guys! I didn't realise that when I was doing it, I worked so fucking hard, week after week after week. I wanted to do a good job, get it right, and I think I forgot about life around me. So when someone says, "Can you take the laundry out for me?" you go, "For Chrissakes! Can't you see I'm busy?"'

For a long time, he was haunted by the failure of his first marriage. Mim was a fellow student at Augustana College in Sioux Falls. They married in 1962. 'My first wife was my high-school sweetheart and when I first saw her, I was 15, and that was it. And it was! But that ended with a personal betrayal... it involved my best friend. And then everything after that was trying to recapture something.'

Abandoned by Mim and his best friend, David became rather jaded about trust. Even now, he thinks strangers are a breeze, compared with people you know well. 'You don't have to invest in strangers. You say hello, give everything, knowing you can walk away.' The effect on his kids, five boys and a girl, ranged in age between 36 and 12, can only be imagined.

'I think my kids have suffered from the fact that I've been married four times. That has put the distance between us.' That they are all based in the US adds a geographical barrier, although he goes over as often as possible, trawling from state to state for catch-up sessions. Has he been a good father? 'I try to be. I don't know. I do the best I can. But at the end of the day I have to accept myself for what I am. Not beat myself up.'

Interestingly, although all of his divorces were bitter ('If it's going to be a nice divorce - don't bother to get divorced'), he's now on good terms with all the ex-wives. Mim, for instance, came over last summer to stay with him and Alexa. 'My ex-wives are all my best friends now. It's a great concept, isn't it?' he exclaims, with pleasure. 'Get a divorce, then become friends.' Friendship was something he overlooked while in those relationships.

'I'm a very selfish person, self-centred... I hope you understand the context in which I'm saying that. I don't say that regretfully or in a judgmental way, it's a fact, and it's something I need to be aware of. I'm not an easy guy to live with... and maybe this wanderlust that I have didn't help.'

The wanderlust has messed up his professional life as well as his personal. 'Because you don't focus on any one thing, you spread yourself perhaps too thin. Opportunities came my way which I passed up on, chiefly the opportunity for setting a priority for my career. So on the one hand I think, "Yeah, that's the way life should be," and on the other I think, "Jeesus, if I'd really just focused on acting, just acting..." But it hasn't been possible.'

There is a flipside to his irresolution. His father, now 86, was a Lutheran minister who went into Berlin during the airlift to work with a refugee organisation, and took his family with him. David always resented the church and the sacrifices it demanded. Back in Sioux Falls, the only time he saw his father show emotion was during the Sunday afternoon concerts put on by the local college orchestra - concerts David was forced to attend while his friends played football.

'My father's a very liberal thinker, but he wears his feelings in a place you can't see them.'

His mother, 'who has one of the quickest minds and is one of the most frustrated women I've ever met', was a promising contralto who abandoned her singing to look after her husband and raise five children. She never let the kids forget it. 'She took care of my father, but she took it out on us. You never ceased to be reminded of how much guilt you should feel for being in the way of your mom's progress. I was very critical about my upbringing. Still am. Their whole religious ethic was a horrible imposition when I was growing up.'

Now that he and his parents have brokered a peace, he's closer to understanding it. 'My father was a really true servant of others, of mankind, not because he was saintly, but that was what he decided was what life was about. That has been his life. And that's hard to deny in myself. I've run away from it, avoided it, been horribly selfish, but I cannot deny that instinct in me.'

Crunch time for David came with the final episode of Starsky and Hutch . 'It was a conscious choice on my part. I was sitting on the top of this mountaintop in a mansion which I built... beautiful house, pots of money... and I thought: now whaddaya gonna do? And that was when I decided to go back to South Dakota, my home state, and made my first documentary.'

Just at the point at which he could have named his price, he made a film about the water table in the Midwest - not a sexy subject, but one which had been bothering him for some time. Over the years, for PBS and educational institutions, he has made several other documentaries on topics which have struck a personal chord: the collapse of Pittsburgh's steel mills, Native American land rights, the industrialisation of farming in the Midwest. His social conscience may have won him friends - he rang Martin Bell out of the blue in 1996, having admired comments he'd made on Newsnight about journalistic ethics - but it certainly hasn't made him any money.

But then, he has never been smart in that way. He and Glaser, who blithely sold their 7.5 per cent share in the show for $100,000 apiece, just before international syndication, were squeamish about Starsky and Hutch merchandise when the public was gagging for it. 'The whole idea of seeing your name on a lunch bucket, a puzzle... I don't like it! It makes my skin crawl.'

During the pop years, he relented and set up his own merchandising company, but made sure that the profits went to Save the Children. Has he finally thrown off his demons? I'd like to think so. At one stage, we are interrupted by an Italian teenager who is nervously waving a piece of paper and a pen. David asks her name and signs his own, below the word 'Joy'.

I ask him if that still happens a lot. 'Yeah, yeah,' he says. 'Well, not a lot. I don't look for it so much any more.'

• David Soul is in Holby City at 8pm on Tuesday 17 April, BBC1.