I had similar overheating problems after changing water pump and t'stat. Turned out I had the wrong heater core hose hooked to the pump. The top hose needed to be coming from the pump. The temp gage would get very high before going cold once the t'stat opened. But I didn't get good heat from the heater core, especially once the t'stat opened. Maybe your problem is with the hoses going to the radiator.