hmmm, My advice is let the dealer have it. I have found this is the best remedy at all times. If I were to take a guess I would think it would be the Idle control valve. If it is malfunctioning it could cause this, but really there is no way of telling without having the car diagnosed properly. Is your CE light on? Is it operating? That is a very strange problem. Good luck!
Happy Benzing,
Brian