My Aztec's fuel guage is acting funny..

When I start the car it seems most accurate and appeares to read accurate.. But when I start to drive it moves around alot and never goes below 1/2 a tank.. When I fill up it always stays between 3/4 and full.. However, once it get's low on gas it starts to bounce around alot.. Depending on the road it can go from 1/2 to full.. This means I am low on gas so I fill up..

What do you think could cause this?