Based on the picture you posted elsewhere, you have a "one wire" fuel sender.
The wire feed going to the meter supplies power (excitation) as well as providing a signal to the meter. The other lead to the "suppressor" filters the sender so that the gauge does not bounce around alot.
I think your problem is with all the rust on the tank. It is not getting good ground connection back to vehicle ground. My suggestion is to pull one of the screws that holds the sender in the tank. Clean it "clean bright and tight. Slip a wire with a ring terminal under it before you replace it. The other end of the wire should go to somewhere on the frame of the vehicle with a "clean, bright, and tight" connection.
If that does not fix it for you, I would remove the sender from the tank and try the proceedure I outlined above.
If you need more help my cell is five four zero, three three six, three two seven four.