I'm dealing with the same issue on my 300SD. The shaking isn't as excessive as yours but the idle can be bad after an hour of driving or it can be fine. I switched filling stations which has helped a little. To test your lift pump, run the cigar hose(fuel return line) into a large bottle. If the fuel flows fast and easily your lift pump is good. I have 5 good injectors from an old engine that idled well, so installing those will be my next attempt to remedy this situation.

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'81 300SD