it's a shot in the dark but the only thing i can think would cause this problem is the timer especially if it's on the boiler the timer has 6 on - 6 off periods and it may be switching on the boiler for a second each period this would be long enough to start pump, fan but not long enough to light burner suggest you review times on programmer and try by altering time to reproduce fault hope this helps
brian, February 2011