This is why I said to measure the voltage when the problem is occurring - anything else is just conjecture and a waste of time. Now you have established that it is definitely overcharging. Sounds like a voltage regulator issue to me. I'm not familiar with the inner workings of the W140 VR, maybe some of the experts here can speak up.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280