am haveing the same problem with mine i replaced the float in the tank with the terminals on tha didnt fix it. the gauge is working i think because if i put the wires a certain way the guage will say its full when its only half a tank. also when the fuel light comes on when the fuel is sloshing about in the tank it sends the fuel gauge to full.
if you find out what the problem is can you please let me no? my next step was takeing the gauge out to replace it see if that will fix it!