Mine also facing the same problem, sometimes the needle will go all the way to full whereas sometime it jus stay at above 3/4.
Have learn to live with it as it does not affect anything except for been a sore eye. Been like that for coming 2yrs liao but everything still working fine. So i guess not much issue bah.
I feel that it might jus be the sensor that is struck at the top part and most likely not the fuel pump as if really fuel pump faulty, then your car would have jus stalled and would not be able to start and move as the fuel is not push to the engine.
I have learnt to live with it. And i believe u should. Jus use the mileage to guage your fuel level. i always reset the mileage meter after every pump so as to double up as an alternate gauge for my fuel. The $500 can be better spend elsewhere :)