I guess you've already checked your ground connection to the circuit. In my opinion,90 % of the electrical problems on these old trucks can be traced to one of two things;butchered wiring or weak grounds. If the wiring appears to be good,my first move would be to test for a bad ground. If it isn't the ground,I'd check the power wire for the gauge,it might have a connection that's nearly broken. If that isn't the problem,I think I'd change the sender. Just an idea...
