I had a very similar problem. The way I figured out I had a problem was by disconnecting my second monitor (both DVI). Once I did that I would get the black screen on boot. Plug back in second monitor it would work. Not sure why, but that's what was going on. I downgraded to 15.7.1 last night and tried 10 cold boots and it was working perfectly. So if you have to have Crimson going and happen to have a second monitor lying around then give that a try, if not, try 15.7.1.
Hope that helps.