I am speculating that your compressor is not cycling. Ice starts forming on the coil. The blower is blowing air around a block of ice instead of going thru the coil. Just to proof it: when the problem occurs, turn off AC but keep the blower running for a while (EC button) to melt the ice then turn on the AC again to see if you can get the cold air back. Another way to test it is to measure the air tempurature from the vent with the blower in lowest speed and the Car is idling at high rpm. Look for compressor recying. Sometime, you need to place a big electric fan in front of the car in order to get the ice to form.