Vacuum leak. If this is an M103 (SOHC), check for a split idle control valve hose, a leaking air flow meter horn to throttle body clamp, and broken vacuum actuator lines.

If the fuel pump relay is quite warm after short runs, replace it, a new one will cure all sorts of ills. Definitely bad if you get occasional stalls while driving.

Check the cap and rotor, a leaking camshaft seal will cause serious carbon tracking and difficult hot starts (crossfire).

Seconding MB DOC, plugs, wires, cap, rotor, good fuel, plug condition.

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!