'06 Ascender fuel gauge

Sometimes the gauge works, other times it reads empty, 10-20 seconds later its on F, 3/4, 1/2, then 1/4 then E. Then it may jump to 3/4 or 1/2 as I'm driving. I have been watching the trip and fueling up completely after a certain # of miles in fear of running out. What causes this and is it an easy fix? My son's '99 passport's gauge wasn't working and we dropped the gas tank and replaced the fuel pump and all. Hoping mine doesn't require this extensive repairs. Hoping to trade within 6 mos. Like the Ascender, but the resale is horrible...