Please Help!

I have a 380SL. It currently stalls after driving it for awhile. When I get to a stop light, I see the Tachometer go up and down. It is revving itself on it's own. I usually put the car in Neutral and try and rev the car. Sometimes that helps, most of the time it just stalls on it's own.

I brought the car to my brother in law mechanic and he has replaced the Idle Computer and the Idle Switch in the car. Still have the same problem.

Can someone give me a clue on what maybe the cause of this stalling? The car runs GREAT! It's just the stopping/stalling that gets frustrating. I really want to get this fixed. I don't want to bring it back to my brother in law without an idea on what the problem is. He'll just replace another part thinking that will fix it.

Thanks,
MadsG