I have some problems with my fuel tanks and Im not sure were to start really.

My primary tank works fine, feeds fuel ok. But the gauge says FULL untill it has about half a gallon left, then it goes from F to E instantly. My second tank is the same way. However, when I switch over to tank2, it sometimes dies on me with a full tank. It will act like its running out of gas.

Sound like anything common? I replaced all my fuses to see if that was a problem, but I really dont know a whole lot about the electric stuffs...