Furnace will light, heat briefly, then blow cold air. Have replaced manual flame roll-out switch, Flame roll-out switch will be tripped by resetting, heat will resume-briefly. Also replaced fan switch. Could not get a original replacement part for the fan switch, but got one within 5 degrees. Neither of these corrects the problem. Furnace is clean, no air blockage problems. Any help you could offer would be appreciated. (Think I might need part number 6263901.) Fred Higgins