Oddly enough, I had this very same thing happen to me this weekend, on the way to a trail...
I would have been a lot more concerned if it had gone to the bottom. After conversing with Tools R Us about it a little (and checking the oil, which was fine) we concluded that the sender was probably going or gone. It started reading correctly a bit later, and hasn't gliched since in two days of wheeling and highway use. I wouldn't sweat it, if it goes permanent you might look at the sender or the connector. Our thinking was that the wire would have to short to ground or the sender itself might have a short in it. Because it wasn't doing it intermittently, I don't think my problem is a wire. If it were I'd expect the needle to go up and down constantly or stay up.
-Spike