a/c cycles between vents, defroster, and floor

I have a 1999 windstar. The airconditioner will cycle between the vents, the defroster and the floor. It worked fine for a long time, then started out cycling only when accelerating up a steep hill, and now it pretty much does it every time i accelerate at all, even when stopping at stop signs and such. It will blow through the vent, which is normal, and then cycle to blowing through the defroster for maybe 10-20 seconds and then will cycle to the floor and finally back to the vent. As long as i'm just driving down the road on flat surface, it seems to work ok. I'm pretty sure it's some kind of vacuum problem, as i think everything is vacuum operated in the climate control. Anybody have any ideas or had this particular problem before?

I have had the same problem, exactly. But now I can't even get it to move from the defrost to the vent otr floor. What could this be? I have checked the movement of the vent doors under the dash and they seem to be okay, it just seems like I don't have enough vaccuum to hold them open and defrost seems to be the position without any vaccuum.
My email is shende@adelphia.net
PLEASE HELP!