Wild variation in temps with thermostat, please help!

So I just got a new leopard gecko. I did tons of reading first so I knew to set up a UTH with a thermostat to regulate the temperatures. I am using a Reptitemp 500R dial type thermostat and the problem I am having is I cannot get the thing to stay where I set it. Yesterday I watched it before leaving for work and it was shutting off at 91 and kicking on at about 88, which was perfect. I came home after work and somehow it had gone up to 101 in the hot hide. I again adjusted it down to where it was kicking on at about 87.5 and climbing to about 91 before shutting off, watched it for a bit and it seemed fine again. This morning I woke up and it was down to 83.5 and still hadn't turned on. Again I manually turned the dial to where it kicked on and it went up to about 85 and shut up, so I bumped it up a bit more and kept doing this until it was shutting off at 91 but at this point I don't know how I am going to trust it when I am gone for 8 hours at a stretch. It is working in that it does turn on and off but somehow it is like it will reach 91 and shut off, the next time it warms it will climb to 92 or 93 and shut off, the next time even higher, so it winds up very far from where I left it. I think I have it set up correctly. I have the thermostat probe taped directly to the UTH underneath the tank, not inside, and the temperatures are being measure by a digital thermometer with the probe resting on the glass in the hot hide. I am at a loss, I just really don't know what to do with this thing. Setting it is fine, it is keeping it there that is proving to be impossible. Has anyone had any experience with these or other similar equipment? I could really use some guidance. Thank you so much!

Sounds like it is drifting. Could be a problem with the unit itself. Is it brand new? If so return it and get something better.

Those units are not that great and being analog are old technology. It sounds like its not working correctly so I would return it and put the money into a better unit.

Thanks guys, since I posted this the thing actually seems to have stabilized a fair bit. I haven't seen it go lower than 88 or higher than 94.5 in the last day and a half or so. I am thinking I may actually be able to get it to work, but knowing that they tend to be finicky is good. I will continue to watch it and if it starts acting up again I will just order a digital one. :)

I havent seen any good reviews on the 500.
Might try another one but I would get money back and get at the minimum a hydrofarm

One of the worst T-Stat on the market, they fluctuate a lot and are prone to fail and when they do instead of cutting the heat off it runs full blast (been there many many years ago)