I am having the same problem with my 1980 300D.
In my case I am very carefully following all the archive advice
regarding throttle linkage settings and vacuum issues.

You are probably doing the same. But I know what you are talking about.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250