I have a Trane XE78 furnace and the flames come on for about 5-10 seconds and then shut off. They re-ignite about 10 seconds later, then shut off 5-10 seconds after the 2nd ignition. I believe they flame on one more time, shut off and then never re-ignite and the blower just runs and runs with cold air coming out of the vents. What could be problem?