I've noticed that a faulty Mass Air Meter usually acts up at idle.
If this unit is bad, or if there's a vacuum leak, it can't figure out the correct ratio of fuel to air, so you see the oscillating idle going up and down.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".