Besides being used to make fire, Propane is used as a refrigerant. The faster you boil it off the lower its temp and pressure become. At the end of the brew day, your cylinders are low on fuel. The pressure is low in the cylinders. The cylinders can ice up and you lose the volume of fuel needed to fire the burners. Then, once you close the valves, the pressure builds up in the supply line to allow the burner to fire momentarily. A 200K burner running all out will eat a little more than 2 gallons of Propane an hour. A 20 pound cylinder holds a little over four gallons of LP. That comes to about 365K BTU per cylinder... For the amount of beer your brewing, get a bigger cylinder or more of the twennies.