I ordered the throttle actuator, and it arrived, but when I took it to the mechanic, he was reluctant to install it since he's not 100% sure it'll fix the problem. Once you install an item, you can't really return it, so he thought it would be best if I take it to the dealer and have them diagnose it, and then he'll do the repair.
I took it to the dealer this afternoon, and I just got a call, and the diagnosis is that there is a vacum leak at the intak manifold. They are replacing the seal at the manifold to see if that takes care of the problem. If it doesn't, then they say the manifold will have to be replaced.
They also said they got codes for the O2 sensor and the mass air sensor, but think the vacum leak at the intake manifold is causing these codes.
I'll find out in the morning if this solves the problem.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".