I had a 77 Bronco that did the same thing... work great for weeks, than randomly nothing. Turned out to be a bad body ground. Battery would have power, but since all the electrics ran through the cab, they didn't ground out so you couldn't turn anything on... seemed like a dead battery. Some lights would kinda work but as soon as you tried to start it just blacked out cause it overloaded the small body ground.... just a though. run a jumper cable from your battery ground to various body parts and see if you notice a difference.

 

I think I've fixed it, the negative ground was dirty, as soon as i cleaned it up the truck fired right up. hopefully thats all it was. So far its been starting fine.

Thanks for all the input. I'll save this page for future reference.