Check your cylinder compression. I had similar symptoms and the issue turned out to be a couple of burned exhaust valves.

Thanks for reply. Compression is within acceptable limits so it seems related to temperature or auto enrichment on Jetronic system.

does it have a oxygen sensor? I have been doing this for years,cleaning carbon off oxygen sensors with steel wool.But I learned my method was flawed. If your exhaust smell rich on the new fuel injected models.Start by removing sensor or sensors keep rear one marked.Soak in gasoline overnight to lossen carbon,lightly brush with tooth brush,and blow dry.RFe install,and enjoy your lost power.
Now sometimes you need to replace one.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt