My wife's '91 300TE has a real rough idle at about 600 RPM while transmission is in DRIVE. It seems to get better when the car is warmed up, but is still rough. It smoothes out when the car is shifted to PARK. Once the RPM is up to 800 or higher, it is smooth as silk.

I had a like problem on a 86 420 sel, changed the motor mounts (they were flat) and it idles much much better with no shake.

I had the same problem. For me, it was the fuel filter and spark plugs. Hope this helps.

I have the same problem on my 86 420SEL and have yet to find the cause. Is it especially worse on really really cold mornings? Have your mechanic do w visual inspection of the motor mounts as Tim suggested, it is pretty quick and easy to do. You can also have your ignition system scoped out. That is a process where they use a diagnostic machine to find problems. It could be the plugs, or it could be the sprak plug wire ends (resistor tips) these are cheap and easy to change. Aside from that you could have a fuel mixture that is too rich.