Have you replaced the throttle body actuator? Mine had this same problem and the TBA replacement fixed it right up. Is the car's computer throwing any error codes? Any diagnostic lights coming on.... maybe irregurarly?

oh, and how many miles is on yours?

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Mark Herzig
1995 E300D 153K
1985 300D 142K (sold)
1979 450SL 122 miles (sold)
1992 500E 127K (sold)
1987 300SDL 132K (sold)
1986 300E 161K (sold)