Hello,
I had what sounds like the same problem with my 300E a few months back. The rear bushings on the upper control links were shot so everytime I would hit a small bump or hit the gas to hard the rear of the car would dance around, much more noticable the faster I was going. I replaced the links and it drives like a MB agian. Mabye you have the same problem. Fairly cheap and quick to fix.
Good luck.