Major problem with thermostat set up, really need some help.

[kiiarah]

I just got a new leopard gecko the other day. When I picked him up I knew I needed a thermostat and UTH, so I picked up both. The thermostat is Reptitemp 500R model and has the dial adjustment for temps. Even though this is a bit inconvenient, it should be possible to find the right dial setting to get about an 88-92 temp range in the hot hide. I thought I had this right, I watched it shut off at 91 (climb a bit higher to about 92) and drop to 87.5 before kicking back on. I left for work and when I came home his hot hide was at 101. Nobody had touched the dial, the thermostat probe is taped to the UTH on the bottom of the tank, the only thing in the tank that could have been moved is the digital thermometer probe (which is resting under the paper towel bedding and is not taped down, is it safe to use tape in a gecko habitat?). Last night I manually adjusted it down until again it seemed to be getting to about 91 before shutting off and turning on around 87 but this morning I got up and it was at 83.5 and still had not turned on. I don't understand why it would be so unpredictable. At this point I am feeling like just giving up on it and ordering another Hydrofarm (which seems to work perfectly on my ball pythons enclosure) but I hate to just throw away the money spent if there is a chance of getting this right. Maybe I am doing something wrong? Does anyone have any suggestions I could try, I am at my wits end here.