The test for the gauge is simply to erath the wire out, upon which it will read maximum, if it does all well and good if not gauge is bust, as this has been done we know the gauge is ok.
The only thing that cnges with temp is the resistance of the sensor, it needs a good earth as thta is what it changes the resistance of, it how much of the "earth" it lets through to the wire that goes to the gauge, so if misreading, there is a mismatch of resistancis between gauge and sender, so I would try a different sender.
tabetha