strange a/c problem in 2001 kia sephia

First off I'm a licenced a/c tech for 21 plus years. The charge in the system is as close to being full as possible. It had a leak in the past but I have just recharged it. The weird thing is that when the car is first started the a/c is nice and cold. but as the day goes on and it gets hotter out the a/c temp. drops from cold to cool. I have checked under the dash and from what I can tell the tempature selector is working properly. I thought maybee it was keeping the heat open slightly, but from what I can tell it is not. is it still possible that heat from the heater core is leaking by? any advice is welcome. the car has about 63,000 miles. not sure of the engine size .....sorry. I''m an a/c guy not a mechanic.