Ted offers some sound advise. I've been through this same thing on a '91 300-SEL.

The problem was a loose front wheel bearing. The gap between the electronic parts Ted mentioned is apparently pretty fine.

Elevate the front of the car, then do the 6 o'clock - 12 o'clock tug on each front tire. One might be loose. If you don't feel comfortable tightening the 126 front wheel bearings, find a shop with a dial indicator and experience using this device.