My 7 yr old TRANE (Model# XE 80) furnace is working intermittently. When the thermostat sends the signal for the heater to kick on, the flames start going without a problem, but after just 6-7 seconds, the flames go out and the furnace tries to ignite again. Then after about the 3rd or 4th time, the fan kicks on without the flames/fire and blows cold air until we manually turn off/on the thermostat at which then all is fine until the next episode. Never have an issue when we manually turn the heater on at thermostat location, and only sometimes it happens(40%) when we leave it on auto. Any ideas/advice would be great!