I would start by getting the tires and wheels high speed balanced, not just balanced regularily.

If that doesn't help, I would take the car to a good independent mercedes mechanic and have them diagnose and check the suspension and related components.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon