Have you considered fuel injectors. May be a longshot but they don't last forever despite what some might tell you. Not such a pain to remove and spray test, not really that expensive to replace either. Also, have you given any thought to checking diagnostic trouble codes. If there's a problem in the Jetronic it will help you immensely to pull codes. Trying to hazard guesses with the fuel injection is a waste of time and money.
also
-Vacuum leaks
-plugged catalyst

I assume you have the m103 powered car, correct?

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'85 300D 450k
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