The lead from the flame detector? Could it be that simple. Ive tested it with the multimeter and registered voltage when it fired up. Obviously when its running its losing something. Seems strange that after a minute or 2 being shut down it'll start back up again. Ill definitely try it though. CheersFave on them is the flame rec lead , they crack and loose continuity, if the probe is brand new , change the lead , I would also , if the customer is ok with it change the spark gen and the ignition electrode , but you could just change the lead and try it .
That normally fixes them . Good luck