Fuel gauges are the combination of a floating bulb in the tank varying the resistance of a wire wound resistor, therein, in series with a meter movement, across 12v-gnd. Problems with senders are not uncommon as the sloshing of the fuel causes the wiper of the variable resistor in the tank to rub over and over the wire of the resistor part. After awhile one or the other craters. If you keep a relatively constant fuel level it may be the resistor wire wear into, but usually, it's the wiper attached to the float that gets it's contact worn off. Common problem due to age and agitation. Since the float is a mechanical device and not lubricated with any sort of oil, it could stick, or the wiper could stick on the resistor as a result of constant agitation. Least likely is a sticking meter but is possible.
Mark