84 300D. When I got the car, the motor mounts were toast, so it was hard to tell how rough the engine was actually running with it sitting on the frame. Did valve lash, engine smoothed out quite a bit at first, but gradually got slightly rougher over the course of a week or so.

Replaced motor mounts, noticed engine rocking rather badly (enough to make the whole car shimmy slightly) at idle, but didn't seem to be missing—idle clatter at crankshaft frequency and consistent over time. Rocking diminishes sharply off-idle, seems to vanish completely around 1000RPM indicated.
Idles at 750RPM indicated.
Ancient engine shocks have not been replaced.
Glow plugs replaced with Bosch pencil-type 2,000 miles/3 months ago.
Starts up instantly.

I know injectors are often the culprit but I did an injector purge (pulled fuel hoses and ran it on pure Power Service Diesel Purge for about 15 minutes, revving occasionally), if anything it made it slightly worse, with the occasional miss. Slight white smoke at WOT.

What is the procedure for running this down?

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)