I had a similar problem last year just before parking the car for the winter. The last fill up the gauge pegged at full for a while, then suddenly worked right for a while. When I fired the car up in spring it was pegged angain and stayed there no matter what I did. Turned out to be the sending unit. If the coil on the sending unit goes bad the needle will read full. There is an easy way to test the gauge and curcuit board, open the wire between the sending unit and the dash board and short it to ground. If the gauge slowly goes to empty the wiring is good and the problem is either the sending unit or the ground. There is a connection inside the trunk, near the trunk latch, that you can seperate and ground to check it out.