Look for vacuum leaks -- sounds as if it's going overlean once in a while.

Suspect anything rubber, and use some carb cleaner or brake parts cleaner spray to spray the connections between the upper and lower manifold and the plastic part between the fuel distributor and the manifold. If the idle picks up and smooths out when you spray and area, it has a leak.

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!