Is it the original sender? Post a photo of it. I'd pull the sender and see if there are leaks in the tube somewhere. Pull the sock off the bottom end, coat the whole tube with soapy water, plug one end with your finger, and blow into the other end. Some later sender, including some repops, don't have the correct resistance and give the gauge a false reading. I've also seen some that don't fit the tank properly. You want the pickup sock to be right at the bottom of the tank without rubbing on it. Bend the tube as necessary to achieve that. The best way is with the tank off so you can look into the filler neck hole to see where it sits. If you've done a few, you can also "feel" how close the sock is to the bottom when installing it by tilting the pickup so it hits the bottom and then guess how far it's off the bottom when you square up the sender to the tank opening.
