I am having trouble with my 8 year old 90+ efficiency Comfort Maker forced air gas furnace. On several occations, I have found the blower running continously blowing cold air. The thermostat is calling for heat on all occations. If I cycle the thermostat to "off" then back to "heat" everything operates properly again. Based on some reading, I suspect an intermittent heat exchanger temperature/airflow sensor or the exhaust blower vaccuum switch. Can you please give me some input before I start replacing parts? Thank you.