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!