The air blower that blows air through the burners and exhaust through the roof stack comes on, runs like it is supposed to for 1/2 minute, and then the glow tube comes on for 1/2 minute and then the gas valve clicks and three burners light with strong flame and while this flame is lit for 1/2 minute, the yellow light is flashing. Then the flame goes out just before the air circulating fan that blow the air through the ducts comes on. It continually repeats this cycle over and over until after several tries, it will click on and heat for the thermostat to shut off. The temp in the house drops 8-10 degrees before the unit begins to put out the heat.