I have a 1973 Plymouth Duster with a 225 slant six. I am having a problem with the fuel gauge. Sometimes it registers and sometimes it doesn't. I tap on the gauge and no change. Car has 98000 miles on it. What are the possibilities is is the sending unit in the tank. What is the best way to test the circuit? I have taken the wire off the sending unit and cleaned it with no change. Any ideas??