Michael, I think this may be a display manager that's not starting consistently. I have got the thing to drop me at a login prompt a couple of times where logging in and then startx worked fine.
Found this in /var/log/slim.log - slim: waiting for X server to begin accepting connections. lightdm displayed the same behavior. Black screen, backlight on, no way to switch to another console, SSH logins appear to work fine. Hope this helps -
