could be the pressure vessel diaphragm, i had similar problem, the diaphragm had a pin hole in it causing the pressure to rise and dump to outside when the relief valve lifted thus the pressure dropped right off and the flame went out. Try dropping the temp on the control as a short term fix till a repair can be sorted, less heat less pressure build up.I did this and it worked, the repair was not too expensive either.
Andrew
January 2009