UberFisch,

This sounds like classic O2 sensor. My car (300E w/m103) did the same thing earlier this year. I'll bet your gas mileage has dropped, too. Do the oxygen sensor (Cheap and easy) and the plugs and you'll be amazed at the results. I'd go with a bosch original type sensor. They can be found for as low as $75.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C