Air cond. switching to defrost and back to vent on grades.

Hello, You have bad linkage to a mode door inside the air ducts. These little doors route the air to the proper location, and are actuated by electronic motors or vacuum (I don't recall which yours uses). When you go up a hill, the mode door swings freely, and closes one passage, when you level out, same thing but reversed. Sometimes these can be repaired without removing the dashboard, but sometimes not. It should do the same thing, or something like it, when you hit the accelerator hard and brake hard.