A/C in the past few weeks turns on and off; last night the A/C was not working and the thermostat was blank. Then, by itself, it suddenly kicked in and started blowing cold air and thermostat showed the temp. This occurs once in a while. Works fine for days, then suddenly stops working for about 8 hours, then resumes cooling on its own.

Some sort of timer will do that but I am sure you would know if you had one on your system. Some utilities now day have installed a use chopper circuit in the A/C section of your system. They usually have to have your permission to do that also. It is a demand reduction method that helps the utilities control energy/demand usage.Otherwise a short circuit or other intermittent problem could be causing the problem.

Check for a loose connection at the plug into the furnace and make sure the furnace doors are securely shut. If that fails start looking for bad door switch or a loose connection inside the furnace electric control area. You are losing power to the transformer someplace.