It could be a lot of dirt in the tank, The gas gauge is reading wrong, or The fuel pump is going. The Fuel pump is mounted vertically on the gas gauge sensor and it uses the gas to lubricate itself. When you get low in level it may cause the pump to not pump as well.
The gas gauge sensors with age can do funny things. When you fill up, how many gallons do you put in? The gas tank on 84-86 Fiero's is 10.8 gallons and 87-88 are 11.9 gallons. My advice is to run it until it starts to act up, Then drop the tank. At that point you can see how much gas is in there, the condition of the fuel pump, and the condition of the gas gauge sensor.
