S10 AC vent issues?

My top 4 main AC vents work sometimes & sometimes they don't, it's like they have a mind of their own. The AC pours out of all the defrost vents and the floor vents. Has anybody ever had this issue?? Please help if you have any ideas.  I just want to get a general idea before I rip this dash apart.  Thanks guys.

sounds like a vacuum issue... all your vents are controlled by a series of "flaps" under your dash that direct the air out of the correct vents. Those flaps are controlled by a vacuum and you may have a leak someplace thats causing certain flaps not to engage... 

Damn vac leak on my under hood ball! a few min of internet hunting. should have done that 1st.