I recently bought an older home that had some fire and smoke damage, which was repaired by a licensed contractor. All new electrical, plumbing,hvac were replaced. The problem is that the up stairs unit runs when thermostat (auto) is set, but after 20 to 30 minutes later the air cools down and becomes warm. The contractor sent his hvac people to fix the problem several times, but the same problem returns. The unit runs but just no cold continuos air. Help..........................

is the outdoor unit shutting down??