Comment 1 for bug 958279

I had this problem too, sporadically, for a very long time. Recently it got quite a bit worse with a new release of fglrx, so I undertook to solve it. What was happening for me is that the X server used the dpms settings specified in the ServerFlags section (BlankTime, etc.) of xorg.conf to put the display to sleep after 10 minutes, then when KDE's power management daemon tried to put the display to sleep after 20 minutes, it caused some kind of race condition in the X server and required a reboot.

My solution was to *add* the following to the ServerFlags section of my xorg.conf:

        Option "BlankTime" "0"
        Option "StandbyTime" "0"
        Option "SuspendTime" "0"
        Option "OffTime" "0"

to turn off xset's dpms handling entirely, and leave it up to KDE to blank the display. Note that all four values default to 600 (ten minutes) if they're not specified! (My feeling is that as long as KDE and GNOME are set to handle display blanking by default in the distribution, then the above values should ship with the default xorg.conf.)

I hope this helps anyone else with this problem. For the OP: you might want to dispable display dimming as well in case fglrx can only handle one kind of power saving request at a time.