A/c gets warm then cold again

You seem to have one of two problems going on. It could be that you're low on freon or your expansion valve (called this instead of H-Valve, Accumulator and so on and so forth), has gone bad. What's happening is the expansion valve is not metering the freon or monitoring it. Your Evaporator inside your vehicle is becoming so cold without the A/C compressor cycling or shutting off that after a few minutes of ice cold air, the evaporator becomes so cold that it literally becomes a block of ice whereby the fan cannot blow through the Evaporator anymore for it's now ice due to the fact that an A/C system first removes the moisture in the air and then cools it. You need to take it into a shop if you're not A/C qualified but for the time being, when the warm air comes out, turn the A/C off for 3 - 5 minutes and then turn it back on. The main problem is the Compressor is not cycling as it should.Could be as simple as a clogged screen in your Accumulator which also is the A/C system's filter and dryer.

Good Luck,

~Stillin~

HI. its best to inspect the switch for faults but, im sure this is a charge issue. i recommend recharging the system asap. it is drying up.