Sounds to me like a vacuum leak.
I had a similar problem, and after replacing the MAS air sensor and then the throttle actuator, the problem turned out to be a vacum leak at the intake manifold seal.
A vacum leak alllows "false air" into the system, and will prevent the MAS air sensor from doing its job.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".