I had a car once that would be fine for hours and all of a sudden overheat. Rodded out the radiator, new thermostat flush ect. Kept on doing it. Turned out to be the water pump. Changed it and all was good. I still don't understand how I could drive hours with no trouble and all of a sudden, in seconds, be overheating. I see you changed the water pump already. Pressure test the system?

Les