There is surely a loose vac line somewhere. The purge line is under the fuel distributor (two nipples right next to each other), so that may be the line for the transmission modulator -- do you have a very harsh shifting problem too? If not, it may be to the EZL ignition module on the fender -- make sure that hose is connected or you will have funny ignition timing.
Can't place it exactly, as I only have a 300E and don't remember a vac tap on that particular runner.
If you cannot find where it goes right away, at least plug it, you are sucking dirty air down the engine!
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!