It may sound funny but you are saying that it is starting to flow hot after some time, so please do this test. When the AC is working correct the condenser in the front is hot, When you feel the hot air flow starts stop the car and open the hood, touch your hand with the condenser.

IF it is cold or hot makes diagnosing completely different. The hiss of sound you told in the first messages, I guess I heard that sound too when the system was over charged with AC refrigerant.

I wish you good luck.