I don't get it, why is it that your heating temperature changes despite you having a herpstat thermostat hooked up? And going over your set temperature? The only reason it would go over that is if that is your snake room temperature. And if thats the case playing with the herpstat shouldnt work. I have daytime highs of 90 and night time lows of 70 in my room, but with thermostats the ambient temp is kept at 80-85, hotspot at 90-92. Even with the drop in temps. Because when the temperature is high, it just puts less power through the heaters with proportional ones, and kicks in less often with the on/off ones. If my room temps get up to 90, my heaters would be barely powered and cooling for the room kicks in.

What heating are you using and hows the herpstat connected? Where is it measuring?