I have a mate with an EA Fairmont Ghia with the climate control.
It has this problem when the AC is on. When you accelarate hard, the vent switches to the windcscreen vent. :shifty:
Once you ease of the fast pedal, a few seconds later it returns to the original vent.
Now i'm thinking some kind of vacuum leak, but I dont know where to check first (concidering my lack of knowledge with EA's)