My uneducated guess is that this is a fuel injection problem (or one of it's inputs). You might try looking through old posts about the 02 sensor or the EHA valve. I know that stevebfl posted some extremely detailed testing procedures for the EHA earlier this month in this thread:
My LAST ovp/eha question, I promise!!!!
Hope this helps.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino