Everyone, I and my shop need some help. I've searched and read every thread here and on other forums. We have the MB service manuals.

Car is a 1984 US 300D. 32K miles (yes, that's not a mistype) The symptom is a hot rough shaking idle which becomes smooth as soon as RPMs get >1K. Cold idle is smooth. And, there was a "stutter" around 1500-2000 rpm; this was fixed with injector pump rebuild/recalibration (see below). Cracking the line on the #1 injector would smooth out the hot idle as well.

What has been done (roughly in order, and after thousands of $$$$ spent):

Diesel purge multiple times
New fuel filters (after every purge)
New motor mounts and shocks
New rack damper bolt
New injectors (tested to ensure equal pressures)
New delivery valve seals
Valve adjustment
Compression test, and compression is good and equal across all cylinders
Injector pump rebuild and recalibration

(The injector pump was clearly messed with by someone before us, because it did not test correctly, so it's good that it was rebuilt and calibrated properly. Cracking the line on #1 injector doesn't have any effect anymore; that clearly was a fuel delivery calibration in the pump.)

Now, after all of this, especially injection pump rebuild, hot idle is better, but still not smooth. I have other W123s, and I know what it should be.

What are we missing here? Any help is greatly appreciated.