I would agree it sounds like a fan problem. With the engine running, unplug the large plug on top of the thermostat / upper hose housing, and see if your two fans come on full speed. There are at least two fan speed settings, when on full they roar, you will know. Then plug back in. If temp gets real hot, you can run the heater to bring engine temp down quickly. Good luck. Are you using MB pink / orange coolant? (Probably if dealer changed radiator)

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro