a vacuum problem at idle. Have you tested vacuum?
Should be at least 15-16 psi at idle.
The intake manifold is a source, if you can get a vacuum gauge hose there (my 280S has one)

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old