Well, without really knowing anything about the car, I would speculate that the catalytic converter could be plugged up. That would limit performance.
Have you taken the car into Donnie's shop (Benzmac) who is one of the owners of this site? He and Bill Woods (the webmaster) own this site and the shop, MB Autowerks, in Sugar Hill, GA. The link is at the bottom of the home page:
It might be worth the trip, and I'm sure he'd be the best MB mechanic you could possibly find in your area.
Heck, I live in Souther California, and I drive 51 miles to a good MB independent that was recommended on this site.