He said he already checked the throttle plate...

I second the spark plug idea...those Bosch Plats are junk, get the proper OEM ones, preferably from the dealer. That alone could very well be causing your problem.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles