I have a Zoo Med ReptiTemp 500R and a ZM Rheostat, both of which fluctuate a good bit. Neither of these devices seem capable of keeping the cool side UTH between 80 and 84, they both want to go much higher, even on the lowest setting (I use the Rheostat for the hot side and the 500R for the cool side, but both are lacking at best). Has anyone else had this problem?? They are both very inconsistent and are bothering me b/c I do not want my BP to become stressed due to bad temps. I was trying to avoid buying a $100+ thermostat, which is why I chose this cheaper alternative...I guess you get what you pay for. Anybody have a good thermostat they want to get rid of for cheap??? Maybe I'm doing something wrong, but they seem pretty idiot-proof. Also, since the problem is them getting too hot, is there a good material to put in between the glass and the substrate? If you have any suggestions, I'm all ears. Thanks for reading this.