I had the same thing happening on my 560SEL, replaced the motor mounts, and it smoothed out for a couple of years. Now it was running rough again, this time a vacuum leak on the plenum seals. I am replacing them right now (a massive job), and one of the seals separated in my hand, with the inner ring detaching from the outer ring. What a mess!

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)