General Maintenance & Repairs - why does my a/c do this?

when i'm going up a hill, it stops blowing, and then when it levels off and goes down hill, comes back on, it's not that chilly of air, but i just usually have the tops off and the windows down, but had to use it the one time it rained really hard. i thought it might be where i don't down shift but i tried down shifting and had the rpms at about 2700 and it still went off

What it more than likely is the vacume valve that is by the computer under the hood, usally when they go bad, it loses vacume an puts the ac else where, Check it there like $5 bucks at local part store in the help isle, Ifnot that a vacume line maybe, Maybe the one that runs into the canister on the passenger side in the fender,.