One question: which distro?
I'm not sure what's wrong. But try this:
When you get the black screen, hit control-alt-backspace. That should kill X. Then check /var/log/Xorg*.log. Or, if it's Xfree, its log files should still be in that folder. Looking through them might give you a clue.
Also check /etc/X11/xorg.conf (or xf86.conf or whatever it's called). See if there's anything odd.
<edit> If all else fails, there's a Wiki on X's site that is for tech support, or something. Or
www.linuxquestions.org, they know a lot more than us. Or irc.prison.net #linuxhelp.