I am working on a old standing pilot Trane BLU120E936B. The burners are in suprisingly good shape. There was some debris on them....like some white little foam balls???...some rusty powder inside them. They were'nt that rusty in the slots. So I cleaned them. It has four burners, and one of the middle ones will lite the length of the burner, then after a few seconds, slowly go out until it comes back to where there is a small ball of flame next to the crossover part of the burner next to the pilot. I adjusted manifold press from 2.5 w.c. to 4 in , recleaned burners, cleaned crossovers, adjusted pilot, checked orifice and it was clean. Any suggestions? BTW...the other burners are burning fine, and from what I can see my problem burner looks fine???