I'd place my bet on the radiator. If its over say 10 years old, just replace it. I also saw a thermostat problem that caused overheating. even a new thermostat is to be suspect. A sticking thermostat will mimic a bad cooling system.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!