I have found that Xorg locks up randomly with both 11.4 M5 and M6 on my annoying Toshiba Satellite U500, Nvidia laptop.
What happens is that the screen freezes and the keyboard and mouse stop working. The only way out is to press the power button, which then performs an orderly shutdown (judging by the HDD LED).
The system logs show nothing - nada, zilch, bokkerol - only a normal shutdown, due to the power button press.
The laptop will run all day in text mode without issues. It is only when I run startx that the problems start. So it looks like 11.4 will make a good server distro…