I have an SR400HD heater that fires up starts heating but stops after about 30 secs. It will cycle again, start to heat and do the same thing over and over. The only thing i notice different is a stronger smell of Natural Gas than usual. In the past when it was heating properly, I dont remember that strong of smell. Any Help be appreciated. We did change the thermistor on outgoing vent but that didnt work. Thanks , Jack