Welcome to Just Answer!The system is going off due to a tripped safety device that is reset-able by cycling the power to the control circuit... by turning the breaker off you accomplished a reset.If the weather has been exceptionally hot lately and you have not cleaned the condenser coils on the outside unit by blasting water between the fins with a garden hose... the unit could easily have gone off on its high pressure cut out switch.
Clean the condenser coil with the power off to the unit and fan motor and other electricals wrapped in plastic to keep them dry.That would not cut power to the thermostat however. But check it anyway.A clogged condensate drain line however will cause the drain over flow switch to trip if you have one of those installed, and that is normally wired to kill the power to the thermostat. Check for that situation even though a tripped drain overflow switch will not reset until the water drains away.Let me know what you find, we can go from there.... as long as you keep rating my answers *positively I hold the question open so that we can continue without any time limit.Phil
Phil, I did give you an excellant rating but I still need to see if your advice actually works so please hold this question open like you said until I know for sure. Thanks. John Picarello
I cleaned the condenser coils (they were very dirty) but still have the same problem.The unit will work for awhile & then just stop. John