Ac occasionally blows hot air

Occasionally the Ac stops running and starts blowing hot air from the side side vents but not the center dash vents I have taken the car to 2 different VW dealers and neither can find the problem The car has been very reliable up to this point and only started doing this this year sometimes it will do this when the car is started or when the AC has not run for a long time but if the problem does not occur and the AC runs for awhile say 20minutes then the AC stays on