I have the belly heat in my T-10 and I have noticed similar problems with the temps fluctuating. I also don't know if it is the thermostat or the belly heat cable, although I do think it is the cable. I noticed the temp on the thermometer are always different than my Herpstat. I've seen the high temps drop down to the 80's when the thermostat is set for 94.5. So I have yet to figure out the problem. I've been moving the probes to different spots to see if that is the problem.