When was the last time you put in a new O2 sensor? That's usually a good place to start when you are dealing w/ the infinitly complicated (and damnable) MB fuel injection (Lamda is a curse-word in my shop)!
I would also make sure my EGR wasn't plugged or stuck open.
Start w/ the cheap and simple first!
Then get out your wallet!
-Matt-

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Currently:
1972 350SL Euro 4spd
1973 BMW R75/5
1981 BMW R80GS
1995 FZJ80 with OM606 Conversion In Progress