Intermittent no start... need help

[White70C20]

So last Sunday I was planning on taking my green '70 C20 for a drive. I go out and try to start it and it acts like the battery's dead, it turned over slowly a couple times and then it just clicked. I left it until Friday when I had more time to deal with it. Just for kicks I decide to try and start it. It starts right up. Thinking it was just an almost dead battery I drove it around for a while to charge it up. It restarted several times over the course of the afternoon, until I stopped for gas. I get back in and try to start it and it turns over slowly and then just clicks. I checked the battery connections and everything is good.

I get a buddy to come jump start me and it fires right up! I'm not sure what the problem is. I'm working on the truck later today and I'd like to be able to fix this problem while I have access to a hoist and tools at my college's shop.
Thanks!