Temp gauge does not stay in middle while driving. Will go from middle to cold & back halfway to middle then back to cold then to middle all in a matter of a minute or two. Never goes to hot & this is as the heater is running. Am orignal owner of car & this has NEVER happened before, temp has always been in the perfect middle position. Been doing research and coming up with wire on sensor, the sensor, thermostat, water pump or head gasket seal. Radiator is full so no leaks. HELP!