is your tank sitting right on a solid surface? You need to have some "feet" on the bottom of the enclosure to let a little bit of air underneath the tank. It could be, that if you're just sitting it on a solid surface, that the UTH is just getting really hot and heating the surface, and holding that heat. a simple test would be to take a piece of paper, fold it in half four times, and put one of those under each of the four corners of your tank. Give it a couple of hours and see if the temps come down a little.

What size is your tank and what type/size of UTH are you using? If it's still getting that hot with your thermostat setting that low, it could be an issue with the UTH.