I am hoping there is a boiler maintenance/repair member who can assist me. I have a Vaillant boiler with a Beckett AFG burner. Nozzle is 1.10 @ 80 degree B. I performed routine maintenance, changed filter, screen on pump, new nozzle, cleaned and adjusted electrodes, and disassembled boiler for cleaning- front plate, top spring plates and top side plates, flue pipes dissassembled, cleaned and reinstalled. Re-assembled without issue. Boiler starts quite and shuts down quite. Problem is that boiler starts quite, runs for about 30-45 seconds and then intermittently sputters. Flame is seen to momentarily go out and come back. Damper moves rapid with a little smell back into the house. I looked online for a possible solution but found nothing other than to seek out a licensed repairman. Tank is underground. Two pipe system. Only thing I can think of is possible blockage or freezing of water in the oil line, which I doubt, vacuum leak somewhere in supply line allowing air entrance, but I don't see any oil leaks to indicate same or possible pump failing to maintain fuel pressure. I don't think the air flow is an issue. Any suggestions to cure would be appreciated. Thanks in advance.