I have a 95 GT I just got the motor back into, recently while driving at freeway speeds my temp gauge has started slowly reaching the right end of the normal bracket but will drop back down to the first quarter of the gauge in a matter of a few minutes if I come to a stop and let the car idle. the upper hose and thermostat housing get warm but radiator is still relatively cool, definitely not 220 hot. I have a 3 core rad, 180° thermostat and fan is on a switch and I’ve tried bleeding it a few times. I know the stock gauge sucks but it would never leave the O in normal, any suggestions? I’m puzzled since usually overheating problems happen at idle or low speeds