[old_skul]

In my case, it was the throttle rod joint on the transmission. Try detaching the rod on the pedal side at the elbow, and moving it (with the car off). If it's stiff and has trouble returning to idle position, you may need to disassemble it and clean it or possibly replace the bushing in there. I lubed mine to little effect from the outside; finally had my mechanic replace the bushing.