Occasional Idle Problem
 

I have a 1993 400E that I have owned since new. Occasionally, I have an idle problem. The problem is this. If you tap the gas the engine revs a little, then most of the time goes back to idle normally. Occasionally, when you tap the gas the idle drops well below where it should, then suddenly picks up speed to prevent stalling. This is my wife's car and can be troublesome in a parking lot as she does not left foot brake. I fear that she may bump someone.

The problem only happens when the car is at operating temperature and has been shut off for, say, 15 minutes. It has also happened on trips when we stop for lunch. Also, in addition to the above, the engine (on rare occasions) will rapidly oscillate about its normal idle speed. The period of oscillation is about 1/2 second or less. This is rare but has happened.

Other than that the car is perfect. It has 127K miles and gets excellent mileage. Spark plugs were changed 5K miles ago and had no effect on problems(s). Problems have existed for a couple of years. They have gotten no worse, yet no better.

Any ideas?