Air Conditioner will blow cold, then hot. Its not freon or relay switch. Ideas?

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For the last few months I have noticed the air would suddenly blow hot for no reason, then come right back to cool. But more recently, it is blowing hot more often can cool. Restarting the car does not fix it. It just seems to happen for no rhyme or reason. We had the freon refilled and that did not do the trick. Then we replaced the relay switch but that hasn't helped either. I am going to take it in to Kia next week but I want an idea of what it could be and what it would cost to fix. Of course my car is just past the 60,000 miles - otherwise it would have been covered under the warranty. (maybe)