Remember - the gauge side works backwards from most auto applications. The light side is "open" when there is pressure, allowing the light to go out, which is common. However and the gauge side is grounded also with no pressure and the resistance increases as pressure builds. Take the wire off the light side and it will not burn, take the wire off the gauge side and it will peg out. So from the symptoms, it could be the wires are touching each other intermittently or otherwise not connecting properly as has been suggested.
Check them closely, those senders are nasty expensive and it'd be a shame to have a spare that you don't need....