It sounds like is evap core is freezing up. I have seen it a few times before including on my wife's Camaro. When you park it for a little bit do you notice quite a bit of water coming out of the drain in the firewall and hitting the ground under the engine? Usually this is a sign of the ice melting. Most of the time it means the A/C has a leak / low on freon and this is causing the freezing up. It could be a bad compressor or switch causing the compressor the over pressure the system as well.