check the ground back there, if its steady at full, means there is plenty of voltage, enough to overcome the bad ground, but as fuel level drops, voltage does, and the needle on the guage does as well, reflecting how much gas you have. The lesser voltage makes the ground issue more of a problem, and as you get closer to empty the guage really goes bouncy -reguardless of terrain or movment. Unfortunatly, you could have the same issue at the ground, in 2 places, outside the tank, and inside the tank on the slider, the full section is at the top, less prone to rust, as the bottom is most prone to rust etc. due to its constant submersion.
obviously check all the exterior connexions, clean with wire brush, put that anti rust goop on afterward or vaseline.. anyway track it down like any problem. But like the others say, wouldnt mess with the inside of the tank unless you feel its that big of a problem.
Iis a huge painin the ass, you have to drain it, remove it, and at that point maybe it will be time for new, bigger, better tank...
next thing you know youve dumped a week and 500 bucks into a guage that bounced around.