It appears to be the screensaver. When that comes on, it crashes. I remember I had this same problem before when I updated X through swaret. It was buggy. And I can't even turn it off right now, because when I go to screensavers in GNOME, it plays a preview of the screensaver, and it freaks and crashes just from that.
Any known issue with the latest XFree86 for slack and screensaver?