From what you have told me it sounds like the a/c works great when its working, that tells me that all the components that actually make the air blow cold are doing their job. Its also tells me that from some reason the a/c is loosing power(a.k.a) "command on signal". That narrows down the list of possbile failures to just a few things. There are 4 things that can go bad that will cause your problem. In my experience starting with the most likely failing part is the A/C Clutch relay. The relay provides power to the a/c compressor, this is electrical component and over time it will wear out and start to intermittently fail. This is located under the hood is the under fusebox. The second item is the a/c control head. That the part part inside with all the knobs and fan setting. These fail for the same reasons as the a/c relay. The last two items I hardly see go bad and dont think they are is the a/c compressor clutch and the a/c pressure switch. If either of the fail they wont work at all. So I would start with that relay. I will attach a picture to help you out. Also a little trick, you swap the a/c relay with another relay from that fuse box. Just make sure the relays look EXACTLY the same. If possible try use a horn relay.
I hope this helps out, I dont know how handy you are with a dvom, but if needed i can give some detailed instructions on checking circuits.