Could be the slosh module or whatever you call it. You might check for continuity in the harness too. I had a similar problem but mine was stuck on full. I'd wait till I thought the tank was almost empty so I could drop it, & when I would test the sender it would test ok. This went on for quite a while, and I never thought to look in the tank. (plus I couldn't really look in there because I didn't want to unplug everything) When I finally looked in there I saw the baffle had broken the welds and was getting stuck under the float.