There is a chunk of pipe inserted into the top of your gas tank that holds a float that positions some sprung copper contacts down a wire. As the float moves with the level of the fuel, the resistance of the wire between the contacts and the end of the wire changes. The wires and contacts become gummed up and cause the resistance value to change erradically. This is why your fuel level needle jumps around. I removed my sending unit and tried to clean the wire and contacts to no avail. It lessened the jumping needle, but in the end, I will have to replace the sending unit just as you will.