My melting furnace has similar problems but with only one burner. After an hour or two the fire pops back into the mouth of the flare. Breaking the gas flow with a piece of sheet metal passing in front of the jet works, but only for a few minutes. My current **answer** is that the pressure in the combustion chamber rises to more than the flow of the jet / gas mix allowing the flame to move back to the jet / orifice killing the venturi effect. ? can you make your exit hole opening larger? or move to a smaller burner tube?