I have an older luxaire furnace maybe 10 plus years, the fan turns on sometimes it will blow heat but most times it comes on blows cold air and stays on for a while most of the time we have to turn the switch to off to shut off the blower, when watching the flame through the window on the furnace it will light up, flame will come on temporarily then goes right out, at times the flame will stay on and heat will blow out what could be causing this?