Could be the fuel just shifting in the tank, mine does that, but mildly. Yours could be the sensor might be bad.
I got a similar problem, my guage read half tank yesterday, (I filled it up Tuesday, I think, and well last night it read in between quarter tank and half tank (closer to quarter tank (toward full mark) and I thought WTF. I got home parked it in front of the house in the road, (yesterday) it read 1/2 tank, then a few hours later (5 maybe) it read almost 3/4 full. Weird, it was sittin in the same spot too the whole time didn't move. Anyone have any ideas on this too?
Also this happened before too, it read past the empty mark almost thought I was gonna run out of gas (no gas station for miles) got to where I needed to be, got in my car to leave it read 1/4 tank left. Read that all the way back into town and I filled it up right away.
