I have had the very same problem with my '82 300SD. Idle is rough and the car shakes quite a bit. Above 1300rpm and above, the car runs as smoothly as silk and strong. The engine is newly rebuilt and as about 5000 miles on it. I have tried just about everything I can think of. I replaced the motor mounts and the transmission mount, put a new rack dampener screw on the injection pump, installed new fuel injectors, re-adjusted the valves, and re-timed the injection pump. My next step will be to have the injection pump tested and rebuilt if necessary since it was not rebuilt along with the engine. I've always had a sneaking suspicion that the problem is fuel related and I believe I'm close to pinning it down hopefully. I'll let you know when I get this one solved.