Fuel sender going bad?

First thing I would do is check if the grounding strap is still in place on your sending unit.

It's pretty easy to remove the sending unit from the tank to check things out. I don't know if the gas gauge works in my "new" 64 valiant but I had to replace the tank due to pinholes. I checked the resistance of my sending unit from empty to full and it seemed pretty linear. So I'm staying with the original because it seems the after market ones do not have the correct resistance range. My original brass float had fuel in it so I replaced it with one from Vans Auto along with the "sock" which was severely deteriorated. The replacement float is brass and looks like the original. Vans has the consistently best prices on fuel components.