In order:

Engine mounts -- if not replaced, do so. Major improvement -- must have nearly an inch clearance between largest diameter of mount and frame.

Vac leaks. Idle control valve hoses are thew worst culprit, but all the vac lines have rubber hose connectors, and they are all probably bad.

Has the cap and rotor ever been replaced? Plug wires? Probably toast by now, again major improvement when replaced.

Injectors -- uneven fuel delivery at idle will cause rough running, cured by new nozzles. Usually cannot be cleaned because the internal filter is clogged. Purge the fuel distributor lines, too.

Worn valve guide seals or valve guides allowing air in around the injection system. Worn guides will require a valve job (typical on older M103s at about 150,000 miles, typically not a problem on newer ones).

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!