71 swinger instrument cluster

I'd say you either have a bad ground strap or the sending unit itself is bad, which is really common. You'll want to yank it out anyway to check the screen on the end of the pickup tube to see if it's still there (mine was mostly eaten away). While you have it out check the sending unit with an ohmmeter on the bench. It should read in the low ohms region and vary as you move the float. If it reads open (hi resistance) anywhere along the travel then it's bad.