Your new thermostat may have a larger internal bypas and therefore more water goes out the exaust rather than through the engine when the thermostat opens (compared to your old thermostat housing). It's critical that the water pump flowrate is "matched up" with the correct thermostat housing internal bypass. If the internal bypass flowrate is too large vs. the pump capacity, the engine may overheat. If the internal bypass flowrate is too small vs. the pump capacity, the engine may not get to operating temp.