Just found this out actually.
My temps in the rack were fluctuating 0-5 degrees from what they were set at. After trying for 2 weeks to steady em, I gave up and posted a thread on here and called Rich over at reptilebasics. After describing my setup and checking the flexwatt to make sure the connections were good and the thermostat was working properly (Temps of the flexwatt were rock steady with the accurite test) He asked about my substrate. Told him it was aspen 2-3" deep and he told me that that was causing it. It was acting like a blanket and causing heat build up that may eventually fry my flexwatt. Well, after switching to paper towels, no more temp fluctuations. Just steady temps.
Funny huh?
Edit: didn't see the newest post haha