When I use the airconditioner, the air flows from the vents, but in just a minute, it starts blowing up on the windshield. It may or may not come back to the vent by itself. I can hit the "floor" button and it starts blowing there, but not for long, it will go back to the windshield. It is blowing cold air, but not where I want it directed.

 

Sounds like you have a leak in your vacuum system. Default setting is DEFROST. Check under the dash to wnsure all vacuum lines are seated tight. Sometimes a passenger gets a foot up under the dash and can dislodge/loosen up a line.