OK. When I'm driving down the road and give it enough gas for the tranny to downshift for more torque to speed up or go up a hill. the ac which was blowing out the dash vents will switch to the defrost vents? Strangest thing I've ever seen. Any ideas??
1992 Olds 98

the door actuators are likely vacuum controlled and defrost is the default position. if the vacuum supply or any of the connections are getting leaky it wouldnt be as powerful to hold in the right position

Possible check valve bad?