My car did the exact same thing. Idle oscillated up and down only when the car warmed up.
It was a vacuum leak at the intake manifold seal.
A new seal solved the problem.
Any vacuum leak, though, is false air, and causes the mass air sensor to hunt for the proper idle. It can't find it (because of the vacuum leak) and the idle hunts 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".