fuel starvation things others have suggested you check... then also check the EGR system.

Our '80 300D had similar symptoms and it turned out to be the EGR system and as soon as it was shut off, the problem was completely solved.

To check this out, just put a golf "T" in the vacuum line... and look to see if the EGR is in fact closed and NOT partially stuck open. Then once you are certain it is shut, drive her and see if the problem is still there.

Sam