The air conditioner in my 2001 Prius starts out blowing nice COLD air, but after about 45-minutes to an hour, it gradually gets warmer and warmer, until there is practically no cooling effect. This happens every day. Day after day. My evening commute home from work is about an hour, so the air conditioning starts out perfectly, but by the time I get home, it's just recirculating the cabin air. Since it starts out cooling the air just like when the car was new, I don't think I've lost any of the refrigerant. The fan works at all speeds. Plenty of air velocity out of the vents. Any ideas??