Peet,
Welcome to the club. I've had the same problem on a 300CE for TWO years. Got a lower limit switch code. Eventually replaced everything except the roof! Still can't find it. My indie has a friend that is a retired master Mercedes mechanic. He has taken on the project as a matter of honor. He thinks the problem is in the throttle linkage although the adjustments are dead on. And the problem is if you make an adjustment or change and run the car, you have to wait until the next morning to see if you have done any good.
If either of us finds the answer, I hope it gets posted.

Anziani
'93 300CE 200K
'95 E4250 122K