The impossible to solve overheating problem

Your description of overheating both sitting still and driving.. I ran the gambit with an overheating prob with my 340. Replaced the thermostat, flushed the engine and radiator and heater core, replaced the radiator, and finally replaced the fan clutch and then the pump. It was the pump that was the culprit. I've read several articles the the fins in the original pump do not have a shroud and that the fluid cavitates within the housing, so I replaced the pump that has the shroud, From Rock Auto, GMB 1201070P High Performance pump. Walla, running at normal temps.

This info can be found elsewhere in FABO.