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
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".