I had a similar problem a year or so ago, but it happened while driving through a garage looking for a place to park. Sounds to me like it's the idle speed control unit and/or valve. My mechanic told me if one...and I can't remember which... goes the other likely does as well. We replaced both...at a cost of about $800.00...and that cured the problem. She now idles at a steady 475 RPM and runs like the proverbial top.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000