AC problem 98 blazer

Here it goes.
The AC works fine for about the first 15 minutes of driving, then it seems like something switches it and it blows into the dash. If you turn it off for about 10 minuutes then back on it works fine again. It also seems to do this in the winter with the heat. It will blow hot air for awhile the it gets cold. Any ideas anyone?

Did you check your A/C compressor????

sounds like it is switching to the defroster for some reason. The a/c comes on when the defroster is on. Can't say why it would do that though....sorry!