Your T-stat could be reading wrong temps because you did not twist the shield back on the probe wire. Johnson controls are very sensitive to low volt interference. I just did a Johnson controls system at True Religion Brand Jeans design facility and it can be very comlicated. Try twisting the shield which is a silver jacket inside the rubber jacket back around the probe wire(back towards the actual probe end) and tape it off. BTW, as Dr. Del stated, you should be using the same wattage Flexwatt as the higher wattage stuff is going to get hotter than the lower wattage stuff.