'88 Fuel gauge
 

First off, I'm new here and i drive a 1988 F-150 XLT Lariat. As far as mods, there arent really any, but it's a clean truck.

Anyways, the fuel gauge constantly moves, and there is no exact reading. When i know its full, it shows beyond the edge of the gauge. when i get down to around 5 gallons, the gauge moves back and forth between full and empty until i get down to around 2 gallons, and then it shows completely empty.
Any idea what the problem could be?
What happens is true for both tanks, they're 16 gallon.

I've learned to adapt to it by watching my mileage, but knowing the exact amount in there would be nice.

Thanks!

Check for bad ground on fuel pump wires.