Ford Focus: strange reving problem

I have a UK 1.8 Zetec 2000 model which as the car comes to a holt the engine
speed increases on its own up to around 2000rpm, only when the car comes to
a complete stand still will the revs return to normal. A few months back it
was much worse it would rise to about 2500rpm then at a stand still stall or
almost stall. The ford dealer replaced a split pipe (not sure which one )
which seemed to cure it for a while.

Anybody know what could be causing this & an easy quick diy fix to save me
the hassle of taking the car in to the dealer?

Thanks in advance

John.

mine does that also. Hit 3500 once. Usually worse when aircon is on,
although often happens when hot anyway. Never happened in winter.