Well, my car developed a very bad up and down oscillating idle. After replacing the engine wiring harness, mass air sensor, and throttle actuator, the problem persisted.
Took it to the dealer, and they discovered a bad seal at the intake manifold, causing a vacuum leak. Interestingly enough, the idle was fine when the car was cold. The problem would only manifest until the car was completely warmed up.
Replaced the intake manifold gasket and the problem was fixed.
My situation might be very different from yours and none of it may apply.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".