Air vents

Hello everybody, I'm still having problems with my air not blowing through the vents. When I turn my a/c on, it blows through the defrost, I cut it over to "Vent", I then hear the baffle close and then I cut it back to "A/C" where it will blow through the vent. When I accelerate, it will blow through the defrost and when I let off the accelerator, it proceeds back through the vent. What in the world causes this? Is the baffle sticking?? Is this possibly in the switch? That's my guess. Anyway, thanks in advance!

It is a vaccum problem if it happens during acceleration. I would check all of your lines and where they are connected to.