AC starts out cold then warms up, but after stopping awhile it's cold again.

Hello, intermittent air conditioning operation is most often caused by a one of a few possible causes: 1) the air conditioning clutch / coil stops working intermittently due to a failure -- the compressor will stop working and refrigerant will no longer get cold continuously. 2) the cycling switch could be faulty. 3) if the compressor runs too much, it can ice over the evaporator core theoretically, and prevent cold air from circulating through the vehicle. Diagnostic testing hands on is required to see what is going on with your Tuscon.