I would double check your temps with another thermometer to rule out if its a problem with your other thermometer or a problem with your thermostat. What thermostat are you using? Cheaper ones, like Hydrofarm, are crap and not very accurate in my experience and take a lot of tweaking. I have NEVER had a problem with my Herpstats and they are always within +/- 0.5 degrees, although its usually closer than that. They are pricey, but worth every penny.

Another possibility could be your heat mat going out. Probably unlikely, though.

But as Dr. Del said, you can insulate it and that will help. Glass tanks are notorious for being a PITA to regulate temps.

If you're worried about humidity, just mist the cage a little more often or add some sphagnum moss or cover part of the top with a damp towel.