Thanks for the reply! Is there a way to test the throttle actuator? Can't seem to find any vacuum leaks so far. Regarding the wiring harness, that sounds like a pretty serious cause. Do the harnesses go bad in these cars? And if they do, does the whole engine harness need to be replaced? Sounds like a nightmare of a job... I mostly know BMWs; this Mercedes stuff is all new to me.

EDIT: Just watched a video on youtube of a harness replacement. Doesn't look too bad. Curious how you know if the harness is bad? Can that be tested as well?