How old are they? I overlooked this on my car and it turned out to be the problem. The reason it baffled me so badly was the similarity to your symptom of not causing problems until warmed up. Ran fine when cold but not so fine when warmed up good.
It's cheap to check.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".