1999 1.8 Ford focus zetec

Hi has anyone come across this problem I have a mk1 1.8 zetec focus that keeps over revving up to 4rpm in between changing gears and coming to a stop, at first it started with a misfire and struggling to run and noticed the pcv pipe had collapsed at the rear of the engine so I've replaced it with a new one and cleaned out the pcv, replaced the spark plugs and the leads and coil pack all brand new and just replaced the idle Air control valve which was a bugger to do, I've also had a garage check air leaks and found that the 2 blanking caps had corroded under the throttle body. replaced with silicone tubes with bolts in the ends.I read on another forum that it could be the sillyist thing like the clutch pedal switch gone bad, can this make the engine rev like this, can anyone shed any light on this please as I've done all I can do and it's bugging the crap out me. Any advice much appreciated in advance.