I have a 2003 Chev Tahoe z71 with 88,000 miles. My Air Conditioner cools fine and then it will blow warm air for awhile. It will then start to blow cold again. Once or twice I have noticed smoke blowing out of the vent for a few seconds.
You should have it checked by a certified AC repairman. You could have excessive moisture in the system in which case the receiver dryer needs to be changed or it could be a malfunctioning pressure switch or thermostatic switch at the evaporator.