the ac blows cold then hot

Have climate control system scanned for DTCs. Possible sensor fault causing improper blend door setting? Just a thought. Was the freon level check with pressure gauges? If so monitoring the system pressures while operating would also give a more accurate idea of what is happening.

I just had the same problem with my 2007 Lincoln MKZ. There was a bad sensor. There is a hugh problem with the location of the sensor. It is between the evaporator and the firewall inside the cabin. This requires the removal of the dashboard to replace the sensor. The part is about $50 and the repair will be about $900. The sensor was allowing the system to freeze because it wasn't allowing the compressor to cycle on and off. It would work fine at idle, but when I would drive for about 20 minutes, the air would go warm and there was no air flow coming from the vents. When that happens, stop and check to see if the hoses under the hood are covered in thick frost and/or ice. As the comedian would say, "There's your sign"! Hope this was helpful for you. I'm an owner not a mechanic.