First, do you have the SR5 Gauges (w/Tach) or the standard cluster?

The most likely thing I would think would be an electrical problem, either a loose connection somewhere between the fuel sender and the gauge, or someplace where the insulation has worn away and is intermittenly grounding to something.

Did this happen suddenly or over time? If it happened suddenly I would sure think wiring, but I suppose your in-tank sender could have gone south too.

My Truck: 1987 XtraCab DLX 22R 4WD 5 Speed Manual
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"Speed has never killed anyone. Suddenly becoming stationary, that's what gets you." -Jeremy Clarkson