Vacuum leak?
 

I've had this problem awhile, but its getting annoying and I dont know what it is.

When I climb big hills on the interstate at around 70 and I have to give it a little more gas, sometimes the air conditioner will stop using the vents and switch to defrost, and when i let off the gas it will slowly change back over to blowing air out the vents. I dont know what this is, but I'd like to fix it. Thanks guys.

Yepz, vaccum leak, ive had the exact same thing. I took it to a stealership because i couldnt find it, i cant remember how much it cost but it wasnt much.

For sure its a leak if you look by your AC reciever/dryer, its a big black bottle passenger side firewall. There is a vacuum line right by there that goes inside the firewall. Follow that and make sure not cracked or disconnected. On my fox body the hose behind the AC controls was loose and caused the same thing to happen.