Likely a bad fuel level sender in the tank - the information for the gallons used readout is calculated by the PCM based on the amount of fuel delivered by the injectors, so this is figured independant of the fuel tank level.
The fuel tank sender reads a variable resistance based on the float level and this in turn is used by the IPC to provide the number of gallons (or "F" if over 18 gallons). If the float is jammed (from overfilling or some other mechanical malfunction) then the resistance will not change and in turn the fuel level display will remian constant.
The bad news is that you probably need to drop the tank to fix this. I have heard that some can be serviced by pulling the back seat/trunk liner to access the top of the unit but I'm not sure if this holds true for a 97 SDV.