Over revving Peugeot 406 1998 1.8 Petrol engine?

I have seen a thread on here with an almost identical problem to mine. When i start the car it appears to be idling just fine at about 1300 revs (ok when cold ) . The problem is that within minutes of driving (i.e when engine is getting warm) once i go into neutral at a roundabout/junction/in traffic it is as though the engine has a mind of its own. the revs shoot up to anywhere between 2000 and 3200 (generally nearer the upper limit) and it continues to rev up and down without me touching the accelerator. It drives absolutely fine otherwise, once i'm in gear it is not loosing power or jerking through the gears. I have replaced the stepper motor (idle control valve) , removed and cleaned out the throttle body with carb cleaner and replaced the engine management coolant temperature sensor. Had a diagnostic done at Peugeot they said no fault was shown and recommended new stepper motor which a mechanic friend of mine has done. I've also tried (as suggested on another thread) resyncronising by turning ignition on 3 times for ten seconds but this hasn't changed anything at all. Please help !!!