I just experienced the exact same conditions in my 92 500E, boiling fluid in overflow tank. With car warm but turned off, put your hand over upper, mid, and lower sections of the radiator. Mine had mid section was much cooler than others (i.e. partial obstruction). As I understand it these radiators have turbulence inducing internals which prevent repair. I have replaced mine. We also noticed that the clutch on the fan had quit working. If the fan spins free when motor is warm, its broke. ( It will always spin free when cold). I have replaced the fan/clutch as well. Only to go for a test run and have the thermostat lock up. Should have replaced it anyway having subjected the car to hot temps anyway. Have now replaced thermostat, radiator cap, changed oil and she's running like a charm again. Check these items before launching into the heater core. As the others mentioned, the core is unlikely. I did recently replace a heater hose...Mine split and was obviously ruptured.

Good Luck.

Dwight

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dwight