Low vac will do this, check the condition of the green plastic lines in the engine compartment up by the brake booster.
Best to get a MitiVac hand pump and see if the system holds vac, too -- might be a simple as a bad rubber connector.
Worst case is the dash vent vac servo gone bad, and it is a pain to replace. More on the order of $1000, though, not $4000.
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!