Probably a sticky control plunger or bad seals. Also can be dirt in the ventrui causing the plate to bind. With the engine off, press it down and take a paper towel wetted down with carb cleaner and wipe it all clean. May fix it.

Otherwise, you will have to remove the fuel distributor and remove the plunger, inspect it, and clean it. If it is scored or has ridges, it will cause the symptoms you have, but replacement of the plunger may also not fix it. May need a new fuel distributor ($$$).

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!