[sourdough]

If it is firing up, heating for a while then shutting down only to reignite then shut down again.....look into the limit switch/airflow. Just went through a similar situation and it was a minor blockage of the exhaust (an insect screen). Your furnace has to have free airflow in and airflow out or the heat inside the furnace builds and activates the limit switch to keep it from getting too hot.