Probably the vac servo on the IP -- exactly the same behavior my 300D exhibited. The diaphram has a small leak.

Do a search on vac shut off or vac cuttoff replacement, details are all there. Easy job, just make SURE it works before you start the engine!

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!