I had a similar experience with my car, it turned out to be some speaker wire I had run that got wrapped around the steering arm under the dash. If you have done any wiring under there for speakers, you might want to take a look.

Otherwise I suspect a loose or slipping belt, or low fluid.

Alon

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