I have a 1993 300E and had a surging idle similar to what you are describing.

Common causes of this sympton usually are:

1. mass air flow meter
2. vacuum leak
3. throttle actuator

My problem in the end turned out to be a vacuum leak at the intake manifold. My crack mechanic figured this out after changing a $961 actuator and a $250 mass air meter.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".