I have a 2001 RUUD Silhouette II furnace My thermostat is set for 74 degrees upstairs but the actual temp is only 64. The furnace kicks on then the flames go on for about 5 seconds then go out. It it will repeat this a few times then the furnace will just stay off for a while. Once in a while the flames stay on and the furnace starts heating the house. Does anyone have any suggestions?