I had this problem on my 85 300 TD also. Not being much of a mechanic, I took it to a local shop that fixed it for $250. They told me the vaccuum lines were screwed up ( I think that's a technical term ). Any way it has worked fine since they worked on the vaccuum lines so that may be the source of your problem as well. Good luck.

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85 300 D TURBO 145,000 mi "liesel" (the hot chick in sound of music)