On one of my rig, I experienced failure and a lot of hang.
Everytime, running top on the computer showed me that xorg was taking 100% cpu.
After seeking on the web, I've found a workaround :-) and it fixed the case :
edit the file /etc/X11/xorg.conf
$ sudo /etc/X11/xorg.conf
in the section device of all of your adapters add the line :
Option "UseEvents" "on"
this solved my problem :-)
update : this helped my system to be more reliable, but hanged each two to three days.
update: as the 1st of december 2011, my three rigs have 15, 15, 35 days of uptime (one is ups backed). The problem was the cpu clocking... I would advice to start low and increase until you get unreliability, then decrease to get reliability!