Sounds like a vacuum leak to me, especially since your transmission shift point has changed--it requires a specific vacuum to work correctly. You should look carefully at all of the small vacuum lines and connections in the engine bay. There are several "multi-connectors" in the vicinity of the fuse box where one line branches out into 3 or 4 different lines. These connectors develop cracks that are evident when when you look closely at the branching point.

I've had so many instances like yours, that I eventually bought 10 feet of vacuum line and replaced every line that I could find on my 300SD. Haven't had a problem since then--though still carry some spare line in the trunck. I'm always shocked at how much of an impact that a small leak has on a diesel engine. Fix it and your power will go WAY UP. Good luck. John Downey