2000 GT Fuel Quantity Fluctuation

Hey all, I've go t a 2000 GT with a fuel quantity problem. I've noticed that my guage moves towards the full mark after some driving. Good thing I always reset my trip odometer everytime that I fill up. It started a couple tanks ago. When I sut the car down and turn it to "on" the guage will essentially reset itself to what looks to be the accurate fuel level in the tank. But after starting and running it keeps moving towards the full mark. Any ideas?