The fact that it runs well at idle kinda makes me think it is not a throttle actuator.

Other than not openning, the throttle actuator shouldn't cause a car to misfire.

I had a S500 that wouldn't drive off at times and would set the ASR light that turned out to be an overcharging alternator.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician