I installed FreeBSD 5.2.1 last weekend. I used ports to upgrade XFree to 4.3, but this behavior occured in the default XFree installed from the CD. I configured X using xf86cfg from sysinstall, and the XF86Config file looks simliar to my RedHat file (which has worked for months).
Issueing startx from my user account usually works, but sometimes the screen goes black, and the system freezes. The keyboard is completely useless at this point, and the only solution is the hard reset button. After the system comes back up, I can startx just fine. No .xsession-errors are to be found. /var/log/XFree86.0.log looks the same after a lockup as it does after a successful startx. (I haven't thought to check the timestamp, I will next time)
I'm not sure how to troubleshoot this without any error messages to go off of. I suspect video (Matrox G400 AGP), only because I cannot think of anything else that is unique to X AND would cause a complete system lockup. Any direction would be appreciated. I can obviously supply any config files needed, but I'm not realy a BSD/Linux/Unix expert yet so really don't know what's useful and what isn't (or even the commands to get the useful information half the time