Also, what are perms for /usr/bin/Xorg? It should be suid root, so the output of
ls -l /usr/bin/Xorg
Should be something like
-rwsr-xr-x root root 2104048 Oct 9 21:28 /usr/bin/Xorg
That is the permissions on my machine (default from install).
John VV's pointer to check the Xorg logs will probably be more fruitful, but I would check this also if it were my system.