i had exactly the same problem in a 1998 fiesta 1.3 and a 2000 focus 2.0 they both revved up when the foot is on the clutch, but would hit idle as soon as you stopped. i must admit its pretty fucking annoyin, makes you look like you cant drive! i had the fiesta looked at and they said it was something called a throttle potentiometer. never heard of it. plus it was 100 quid and the motor couldnt have been worth much more! lol. aside from that, be with removing your iscv and giving it a clean.

Cheers! neil