Let me guess you both have 97's
I forget what its exactly called but by the fuel pump, the housing has a float that reads the gas level in the car...that floats up and down along some electric contacts...the circuit that reads that and send the info to the gauge is craped out. A new one is an insane amount of money or so ive heard. Ed can get you a newer housing and you can swap your old pump into the newer one. I have one sitting here but never bothered changing it.
My car has done this forever...I just got used to it. Once I get under a half a tank it starts going haywire...if I run it till its almost empty then it starts reading the correct fuel level again.