This happened to my car a couple years ago. The wiring checked out ok so I replaced the sender and it did not fix the problem. I ended up replacing the guage, and that was the ticket.
The temp and ful gauge are a one piece unit (the cluster is broken up into three parts behind the bezel). I paid $115 with my supplier discount, but that was for my '95 cobra with the white gauges. It's not hard to do. If you end up going that route after verifying the wiring like they suggested above, I suggest pulling your cluseter out of the car and doing the work on a kitchen table. It's kind of delicate.
Feel free to shoot me an email if you have any questions.