It could be a number of reasons. It may be as simple as debris on the condenser not allowing air to flow sufficiently through the condenser fins. It could be a bad compressor. It could be insufficient refrigerant. It could be a blockage in the refrigerant system. You need to take it into a shop and have a system performance test run on it. While you could do this on your own with a manifold gauge set, it won't do you a whole lot of good if you don't know what the numbers on the manifold gauges are indicating.
One thing to do is to check the A/C Relay. Mine wasn't plugged in once and it did that.
Probably because the ac needs to be recharged..
low freon
a fuse could be blowed or your ac needs to be charged.(or both)
Try getting it recharged.
Probably low on freon.
check the fuse
refill the a/c, it will blow hot air with no refrigerant in the vehicle.
"http://wiki.answers.com/Q/Why_does_the_rear_ac_in_97_chevy_venture_blow_hot_air_when_the_front_ac_blows_cool_air"
The system probably needs a recharge.
get youre a/c charged
it could be the heater core and ac housing unit that allows hot air to be mixed with the cold air or it could just cancel out the cold due to the motion of the car