Why does the vents switch to deforst when you increase speed of a 95 Chevy Astro And when you take your foot off the gas pedal or when car gets up to speed then the vents return how can I repair this?

I had the same problem with my '97 Astro van, the plastic vacuum line from the PCV valve to the vacuum cannister underneath the front end had broken. It might not be that vacuum line, but you do have a vacuum leak somewhere.