The float sensor in the tank tends to go out, especially with a car ten years old. My 1999 is already shot below half a tank, I use the trip odometer to judge when I need to fill up. The part itself is integrated with the fuel pump, GM won't sell you just the float and sensor. Most auto part stores won't do it either. Another thing to check, although not as likely, is the connector for the instrument cluster. Sometimes the contacts can bend and wiggle loose with time, making intermittent contact. But if this is a constant thing, i.e. it always goes to the same level around the same time as you fill up, then I'd put my money on the sending unit.