turning on your headlights will increase the load on the alternator. i'd start with the charging system...check all connections and the voltage regulator first. what's the voltage at the battery terminals with the car idling? and with it idling with the lights on?

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1973 280 - Current Project Car
1979 240d - 100% Stock
1982 380sl - 100% Stock
1985 190e 2.3 - Heavily Modified