'70 fuel gauge-weird problem

The contacts in the ignition switch are getting burned which increases the resistance. This lowers the voltage supplied to the fuel gauge to the point that there isn't enough to run all the way to the sender in the tank and have a signal back at the fuel gauge that will read more than empty. Since the rest of the gauges run off a solid state regulator at the fuel gauge and their wiring runs are lots shorter, they work. By bumping the switch a bit, you are moving to a clean area on the contact and you have full voltage to the fuel gauge and voila, it works again. Ran across this more than once. Let me know how this turns out.