2001 Ford Windstar loss of A/C over 3,000 RPMs

My A/C or (Air FLOW) in my van has diminished and switches from the passengers vents to the windshield defrost vents when rpms go over 3000 or when accelererating fast or up hills. The air flow is redirected to the windshield vents untill I ease off the accelerator and let the rpms fall below 3000 and then the air flow or A/C switches back to the regular passenger vents. I live in the desert and this is very annoying not to have the air constantly blowing from the proper vents. No loss of coldness or engine performance; just a short time with no air. I read somewhere that it might be a crack in a vacuum hose that causes this vent to switch. Any Help would be great!