Glow Worm Fuel Saver 55F?

My Glow worm will fire up for 2 or 3 minutes and then cut out. It will continue to spark and try to light the gas but it doesn't. We have had 2 different engineers out to look at who have said that as it has several safety features on it it could be one of about 3 or 4 things that are causing the problem. The pcb has been replaced but that has made no difference. We have tested the air valve and the power to that seems to cut when the gas turns off. Does anyone know how this works and what it controls. Any suggestions on what might be the problem would be a great help. Thanks