The sage continues...

I used the propane torch and tube method to search for vacuum leaks but didn't find anything.

I cleaned the MAF sensor with MAF cleaner and it worked!!... for a few weeks.

I replaced the MAF sensor assembly and it worked!!... for a few weeks.

The oscillating idle speed and idle-down-the-road-at-40mph problems started again a few days ago. This morning I decided I'd take it into my independent Mercedes shop tomorrow to let them sort it out.

But the next time I started my car this afternoon, it was fine again.

I don't think it's a throttle position problem since the when the engine is revving up and down when I'm not moving, the engine is bouncy and rough. If I was just revving up and down with my foot, it would be smooth. Also, when I'm driving, it doesn't idle up and down but only idles high.

Any new ideas? It's driving me batty!