Could be a number of things, I would think that if the compressor were slipping, your belt would have shown some serious signs by now.
Your compressor could be getting weak or maybe you're just running low on refridgerant. The #1 thing, start cheap and work your way up. If you have a friend that works at a dealer, maybe he's got access to an a/c machine and would be able to help you out?
You could always put some a/c oil dye in your system too to make sure there are no leaks. Compressor would be a last resort.