I've used that same thermostat on up to two heat mats at the same time, with no problems. Are you adjusting the thermostat setting based on an independent thermometer (or heat gun)? I did find that the temp reading on the thermostat dial itself will not match the true hot spot temp, so I just sync it to a separate thermometer probe (or gun). Another point that has been mentioned is that the on/off stats don't do well in a room with highly fluctuating temps. My room temps only vary 3-4 degrees.