Your issue is likely ignition related. The coolant temp sensor won't have any effect because more than likely you are not running in closed loop. (Have you checked your duty cycle?).

I understand you said you did a "full tune up". When? What parts did you change? Did you use genuine MB parts? Bosch? Other? People tend to cheap out on the distributed cap. Bosch is $100. Pay it. The other ones last five minutes. If you take off your distributer cap and see some black charred carbon on the points, maybe your coil is running a bit too hot.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2