How long have you been running it? I'm just wondering because on a new setup, the temps will fluctuate a lot until it all settles down. Other than that, I don't know.

I have a follow up question though:

Why should you put the t-stat on the outside? Seems like air would affect the temperature readings and make it hotter. I'm using a UTH though, but have never had any issues with the temp getting too high.