Chevrolet - Silverado1500 :: AC Cuts In And Out

My dad recently asked me to find out what is wrong with his A/C unit on his truck. He has a Chevy Silverado 1500 LS. it's a 2002 model. um.... anyway, he usually blasts his A/C around 3 to 5. the a/c setting goes from 1 thru 5, and off. so i was thinking that since he uses the a/c so much that he's gone and broken something.

In the middle of summer so i suspect the heat has something to do with it. some days the a/c works, sometimes it doesn't. although i've ridden in the truck a few times since the problem appeared, I've noticed that the a/c almost always works when we pull out of the garage or when its cool enough. and never works when we come out of a store or somewhere to the truck that's been left out in the heat in the parking lot.

There's one more thing. i haven't noticed it yet but this comes directly from my dad. he says that sometimes he'll just be driving along and he'll hear a 'clunk' noise and the a/c will start working again. How to fix this problem. I assume this is something very common. and would it be easy for me to fix? i know more about computers than i do cars, and I've already taken apart 1 laptop and fixed 2 others. I feel fairly confident taking part of a car apart.... er, truck in this case.