2001 Expo A/C

I have a problem with my A/C. If I turn it on cold when I am stopped it blows just fine. When I take off or anytime I accelerate it sounds like a flap under the passenger side dash closes and the A/C stops blowing out of the vents and blows out of the floor and defrosters. I have to turn the A/C off and on a couple of times to get it to "stick" to where it comes out of the vents again. Any ideas?

Sounds like a vacuum leak. Start by checking the hard plastic line under the hood on the passenger side. It goes behind the battery and eventually goes thru the firewall on the same side. The line is prone to cracking.

Sounds like a vacuum leak. Start by checking the hard plastic line under the hood on the passenger side. It goes behind the battery and eventually goes thru the firewall on the same side. The line is prone to cracking.

How does come into play when accelerating?

When the vacuum source that controls the different positions of the heater and AC goes out, it reverts to the defroster mode and the vent onto the floor. When you accelerate the vacuum drops and you lose your AC. There should be a vacuum cannister under the hood that holds the vacuum so that when you are accelerating you still have enough to control the heater/AC.
It could be leaking or like has been said already the vacuum source could be compromised.