not so accurate guage

There are 2 different designs of sender found in a-bodies. I'll bet a quarter you have the sender with its electronics in a square enclosure. There is your problem. That one collects debrie and screws itself. Another possibility is a saturated float ( fuel got inside it ). In any case...
Buy a new sender. Dont buy the cheap one with the plastic float.
All reprodcution senders have the half moon enclosure. The new sender will include a new rubber seal and lock ring. Do use the new seal with no sealant, just clean surfaces. Toss that new lock ring in the garbage and reuse the original lock ring.
Dont thank me , thank FABO