Gas Gauge Innacurate

I have a 1999 Grand Am SE and when i bought it, it had about 75,000 miles on it now it has 127,000 miles on it and i have had problems with the gas guage misreading. After about 2 years of owning it the gas gauge once my gas level gets below a half a tank it reads whatever it wants. It will say full sometimes, empty on others are just stay around half a tank for miles than it should. Also when my gauge did work correctly it would seem that my gauge would stay above the half more for much longer than it would once it was below a half a tank.