AC goes into defrost mode when accelerating

It's been a while since I've been in here, so hope everyone is well. I've noticed that when I have the HVAC blower on and set to blow through the dashboard registers, then push the throttle down to accelerate (up a hill or away from a stop), the blower door will divert the airflow to the windshield. This says to me that I'm losing vacuum when I accelerate but don't know where to look. I just replaced all the vacuum lines abt 6 months ago due to tranny fluid contamination, so normal wear shouldn't be the reason. Any thoughts or experience in dealing with this? Thanks in advance for your help!

Added 9/18/06: This only happens when it is in blower mode. When the air conditioner is on, it blows normally no matter what I do to the throttle. Any ideas?

I had a similar problem with my 97 Jimmy. Into the dealer and they replaced the HVAC switch in the dash. It worked for awhile and quit again. Dealer eventually changed the defective switch and it is working again.