If you never register 3/4 tank etc.. the problem is in the gauge.
Inside the gauge is a bimetal strip ( bimetal is 2 kinds of metal laminated together ), with a wire heater coiled on it that makes it warp to move the needle. Eventually that strip looses it original temper, or seperates or it just cracks. End result is a pointer either here or there. Hope that makes sense. I'd say buy another gauge, with your fingers crossed.