If it blows cold for awhile, this doesn't appear to be a low freon issue. Have you checked to see if your A/C compressor clutch is functioning after it stops blowing cold? It should be cycling on and off every few minutes. If the compressor isn't running, you'll need to figure out why. It could be something like the compressor clutch relay having an intermittent in the coil windings which causes it not to work when it gets hot.