Bad Gas Gage

I have a 1994 Chevy Astro EXT, The Dash is ALL Digital, Gas, Speed, Battary, temp, and Oil Psi. The gas gage shows full when it is and I'll be driving all around and it will slowly drop, I dont like it to get past 1/2 a tank. So then i park and turn it off and when i get back in and start it back up it says that the gas is FULL when 10min. ago it said it was @ 3/4 of a tank. Anyone know why? Please Email me @ [email protected] or [email protected] and put it the subject field "ASTRO" so I dont Delete it or send it as Junk Mail. Thanks for the HElp!
-Jimmy
Walgreens, The Pharmacy America Trust!