With both my old GT220 and my new GTX660, the screen will freeze when starting up KDE using the 310.19 driver. It will only happen if the screen has been asleep for an extended period of time. I usually notice it overnight. Today it happened when I logged into KDM at 6 P.M. having not used the computer since noon. I can log in via KDM just fine. When KDE starts, it shows the splash screen. But before the last two icons come into focus, the screen freezes. It seems like KDE is continuing to boot because after a few moments, I’ll hear the startup sound. I have to use crtl-alt-backspace to get back to KDM. From that point on, everything works normally. Reverting to 304.xx fixes it.
I’m using Slackware64 14.0 with the default Xfree and KDE 4.9.3. The attached bug report was generated without the extra X logging. I ran the bug report after bringing down X via “init 3” as root.