55 gallon? I'd say maybe your UTH isn't big enough for the tank, that's a pretty big size. Most of us on here use human heat pads; they run about $12 at Wal-Mart, and as long as you get one that doesn't automatically shut off, they work pretty well for heating the tank. Generally overhead lights can suck out humidity and require a lot of misting.
It might also be your thermometer....the stick-on kind can be pretty faulty, as can the stick-on hygrometers.