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.
1995 E420 - retired
1986 420SEL - retired