Focus with sticking throttle

Got a European (British) model 1999 Focus Ghia 1.8 16v petrol.
A while ago, it developed an intermittent fault. Effectively, the throttle would appear to stick. You're be driving along, foot off the
the throttle, yet it would still be gradually accelerating when it shouldn't be. So put the clutch down and coast, but rather than the revs drop to near tick over, it would rev up to around 2000 rpm and stay there.
The peddle wasn't stuck down, and the cable doesn't appear to be a problem, yet nothing you can do will make the revs drop. That is until you come to a complete stop. Only then does it drop back down to tick over. And while ever at a standstill, you can then rev it with no ill effect, but as soon as you start moving again, it's back up to buzzing around 2000 rpm again.
As an intermittent fault, it was liveable with, but unfortunately, it's been stuck this way for the past 3 days, and it's getting rather disconcerting having to brake every few seconds to stop myself piling into the car in front. And it's probably using more fuel in the process too.
What does this sound like might be amiss? Has anyone ever heard of such a fault? Does it sound expensive?
Cheers!
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could be your ISCV - Idle Speed Control Valve - had the identical problem on a 1.8 escort zetec - might need replacing - i managed to sort mine out by removing it and cleaning it up

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Almost certainly a valve or sender. That's what computer codes are for, to diagnose this kind of problem. You need a dealer or properly equipped repair shop to read them, because it requires specialized equipment. The fix should be fairly cheap and simple.
I know that I was impressed at how nicely my 2000 Focus ran after the intake system was cleaned.
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