I have an 8yrs old Trane furnace which burners fail to ignite after 1 or 2 complete cycles of heat. Typically, I have to wait about 6 hours before I can make the furnace come on again. When it doesn't work, a flame comes on, but then there is click soud and the burners fail to ignite. Then the furnace just blows cold air. When the burners ignite and the furnace work the cycles are completed as set by the thermostat, but I don't know why the burners fail to light at times. Any suggestions are appreciated!