AC blows hot air after desired temperature is reached
 

Car: Audi A6 2005 3.2 Quattro

In the summer as I start driving the AC works just fine but shortly after the engine reaches it's optimum temperature the left + right passenger units as well as the windshield unit start blowing very hot air. Turning the AC on or off doesn't fix the issue, lowering the AC to the lowest temperature also doesn't do anything. I brought this up issue up to my mechanic and he attached a computer to the car and reset the system that controls the air. The AC immediately started working as intended but shortly after driving for 15 minutes the hot air came back. I returned to the mechanic, he checked the computer for errors but nothing came up. He reset the system once again to get it working. Shortly after driving away for the second time the hot air returned.

Has anyone had a similar issue or knows how this can be fixed? Winter time is not that bad since I will be using the heat most of the time but I'd like to be prepared for the summer as it was a torture driving on a highway with no AC in hot weather. Having windows down does only so much.

Any suggestions are much appreciated.