A/C problem

[sli240sx]

What's up guys have ac question for you, my ac seams to not blow as hard at times? I don't understand what could be the problem. When I'm driving on the highway for around 20 or so min the air coming out the vents slows down? I try to turn it up like to 3 or 4 but it just makes more noise like its working but it doesn't come out the vents, just very soft. Is there a flap our something that could have gone bad or something? Once I get of the highway it starts working like like it should again its weird. If anyone has an idea what it could be that would help me out.