the gague is just a volt meter and is reading a voltage across a variable resistor inside the tank. Resistance changes as the level goes up and down. If the gauge stops working, (only part of the time) normally you have a bad wire or connection. Look by or near the tank, that is the normal spot. Could also be the plug at the tank float. Normally if the float resistor is the problem, you will read when the tank is full or empty but not both. If your reading is random and not related to a specific level-- you have a wire connection problem.