astra 1.8 1998 c18xe engine yoyoing?

once the car is moving it drives fine.when slowing down from speed its fine until you slow right down for say a junction then there is a severe engine judder,this is when the engine is at low rpm. also when it is at rest from a slow down the engine is yoyoing constantly.the idle control valve has been stripped and cleaned as was the throttle body.we have a spare idle valve but when shook lightly there is a small clicking noise, have been told this means its duff? any help greatly appreiciated.