After setting the thermostat, and getting initial start/heat, reaches set temp, shuts down, ... then doesn't come back on. If I turn it off/on at the thermostat, it comes back on. Sometimes it works correctly all day, but then it stops cycling. Another occasional problem, it will cycle on/off 30-45 seconds and keeps doing that, even when I turn it off at the thermostat. Sail switch works, so circuit board or what? HELP!