Heat/AC only blows through defrost, then after 10 minutes only blows through vents.

What is your question?

When I start my car, the heat or A/C will only blow through the defroster, then after 10 minutes it gradually switches over to the mid level vents and will not switch back. It does this whether I select "defrost" or "vents." If I select "lower" it will blow on the floor, and it will switch back to either the defrost or vents (regardless of which I select) depending on how long the car has been running and which one it was blowing from before. Any ideas what is going wrong? Is this controlled by vacuum in this car or is it electric? Thank you for your help. Joe