Thank you for your reply.
I have had this 1991 car since new and know it's history.
Car did not have a rough idle until the injector pump seals were replaced.
Two other MB mechanics have now told me there are some delivery valve needles (pins-I think I have this right)/springs in the injector pump that can be damaged or improperly reinstalled during the procedure which can result in a rough idle condition.
One of the two mechanics has since worked on the problem and has concluded it is an injector pump problem.
Is there anything unique about the 350 SDL injector pump delivery valve needles (pins) and springs or are they common to the other MB injector pumps? Can the springs be accidently installed upside down?