4 nights ago I woke up and my thermostat read 65 dg. but the temp was set at 72dg. I tried to raise the temp so it would kick on but it did not work. So, I turned it to ON and it kicked on and worked fine. I then turned the power switch off and turned it back on, switched it to Auto and it worked fine. The same exact thing happened this morning. The furnace is 12 yrs old. Number 1. why is this happening. 2. How do I fix it so it does not happen any more?