Why does the AC on my 1999 Cavalier blow warm air when stopped or idling, but when I drive it blows cold air.?

Question

The longer and faster I drive the colder the air gets.  In fact, it gets so cold that I have to turn it off for a few minutes to warm up. I can occasionaly feel cool air blowing slightly beneath the dashboard by my legs even when the AC is off if I have been driving for awhile at freeway speeds.  What's wrong?  Is this expensive to repair.