I've not had any issues with Ultratherms. I use them a lot - in fact, they're the only ones I will use now.

How long ago did you do this? It's possible the t-stat, heat pad, and accurite all just have to adjust. It usually takes about 2-3 hours for a heat mat to reach optimal temps when first plugged in.

Your aspen may be too thick as well - hard to tell from the pics. I use disposable cage liners instead of aspen in my tubs.

Accurites, while much much better than the crappy analog dials you find at Petco or somewhere, are not 100% acurate. I have 3 in three different tubs all in the same rack and they are all about a degree apart....

Hope that offers a little insight for you!