If you are still on the original sending unit, it may have gone bad at the top of its range. Replacements are around $200 from the usual specialist suppliers. Only you can say if it is worth it to you or not.

I have the same thing happen after changing the pump. The float rod is raising so high that the wiper arm is going off the resistor scale.

This creates a open circuit till the gas level falls.

Good news is you have just might a little more gas at the bottom Or it will still read correctly. So you won't run out of gas because of the gauge.

I live with it because I don't fill it up real often. It lasts for about 15 miles and its too much work.

If you don't mind, then it doesn't matter, because if you don't have a mind, it doesn't matter!

Phil

just had this problem myself. i think the fastest diagnostic is to pull the access panel in the trunk (sedan) or behind the back seat (wagon). check the wire connection on the circular plate on top of the fuel tank under the access panel—if its all corroded, or if the wire has corroded off completely as in my case, you're due for a new sender. don't even bother attempting this job without the ipd bung nut tool. it is worth its weight in gold (which is much more than the $30 you pay to get it, including shipping).