XC70 Air con operates intermittently

[JRC19S]

Hi I wonder if you can assist re my XC70 airconditioner. When I start the car from cold (outside temp 80 F or 25-30 C) and run for about 1/2-3/4 hour everything is fine. Then, the system starts to blow hot air. So I turn off the a/c for a minute or 2; then restart and it goes cool again but 5-10 minutes later it starts to heat up...

seems as if the compressor is working but maybe there is some sort of thermostat in the system, or a blockage...any ideas ?