When was the last time the power steering fluid was changed? If all of the above doesn't work, flush/change the fluid. I had an impossible-to-kill shimmy in my W140 that disappeared after a PS flush & fluid change.

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles