I agree with Randy. It's most likely a fuel problem. I had the same symptoms with my '85 380SE a couple years back. Replaced injectors, fuel head, lambda tower, and enrichment compensator. Cured the problem. Also, make sure mixture is set properly. If you've got a lambda tester then you're set. If not, leave this one to the shop.
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'79 6.9 (SOLD)
'93 300E 2.8