somebody plz help with weird ac problem

hello everyone. i am in the navy and work in a show full of mechanics and no one can tell me for sure what is wrong with my ac. i have a 2001 suburban and for the most part my air conditioning works fine. however when i have been driving for a while and i end up getting stuck in a traffic jam my ac wont work as usual. the air will still be slightly cool but humid. very uncomfortable to the point that i would rather have my windows down with the dam 115 degrees temp out side. same thing the other day i drove on the highway for bout 2 hours and the ac worked fine but when i stopped at the micdonalds drive thru same thing happened with the humid air. i know its not the outside air coming in because like i said the temp outside is real hot and the air that will come thru the vents will be a little cool just humid though. when the ac messes up due to going real slow or going idle, if i end up going back on the freeway going fast speeds the ac still wont start working for sometimes about an hour then maybe it will start working depending on how it feels. also i dont know if it makes a difference but i have been noticing that the force that the air comes out of my vents changes sometimes. if i set the ac to a 4 sometimes it will feel like a 2 or 1. i have already checked my clutch and it is spinning fine and i changed the ac relay but to no avail. if anyone has heard of this or has some help for me please help. thanks for any help.