I have a Pentair Mastertemp 400 natural gas heater that keeps cycling on and off. It will heat the spa water up to about 97 degress then it will start cycling on and off every couple of minutes. I have the temp set to 104 degress but it will not reach that temp becuase it cycles. The HLS LED lights up on the back of the control board when it cycles off. I have tried replacing the high limit switch and the thermo regulator. Neither part fixed the problem. I have noticed the exhaust starts to get hotter before it cycles off. Does anybody have any suggestions on what to check next?