Now that I recall, my car had similar surging, and the culprit was a vacum leak at the intake manifold.

Sooooooo, the three things mentioned are almost always the things to look at for a hunting up and down idle or a surging problem.

1. vacum leak
2. mass air meter
3. throtte actuator

__________________
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".