Hmm, have you lifted that side of the tank enough to get your hand under there and check the UTH to see if it's still working? I once had a UTH, fairly new, quit and die on me all of a sudden.
Also, have you used another measuring device, such as a temp gun, to make sure your thermometer hasn't quit on you?

These are the only two things that come to mind at the moment. Normally temps don't just drop and stay down without reason.