2002 Buick Century AC blows cold then starts blowing out ho

I am puzzled about what could be the problem with my air conditioner. It starts blowing out cold air and then starts blowing out hot air. Especially when the temperature is hot and humid outside. When it is cooler outside, it seems to work longer blowing cold air. Took it to the dealer and they said they cleaned the condenser and recharged the freon. That lasted about 2 days before problem re- occurred. Now it is just blowing out hot air.