Running 3.2.0-55-generic, my system again hung this morning but the evidence is increasingly pointing to either the X server (Xorg) or the video device driver as the culprit.

As earlier, the keyboard became non-responsive (except the kernel magic keys) and the mouse nearly non-responsive, the mouse cursor moving in hesitant steps as the mouse was moved. I logged into the system from another machine on the LAN to examine the running processes. avconv was converting a video file (68% CPU), kworker/1:0 was consuming 62%, and Xorg 52% on this Intel Core2 T5500 dual core mobile CPU. Suspecting the X server from earlier observations, I examined its log file (/var/log/Xorg.0.log):

...
[ 41368.758] [mi] EQ overflowing.  Additional events will be discarded until existing events are processed.
[ 41368.772] 
Backtrace:
[ 41369.118] 0: /usr/bin/X (xorg_backtrace+0x37) [0xe3e837]
[ 41369.118] 1: /usr/bin/X (mieqEnqueue+0x223) [0xe1cb83]
[ 41369.118] 2: /usr/bin/X (0xcb6000+0x4ccd5) [0xd02cd5]
[ 41369.118] 3: /usr/bin/X (xf86PostMotionEventM+0xf9) [0xd43d99]
[ 41369.118] 4: /usr/lib/xorg/modules/input/evdev_drv.so (0x40e000+0x3c3d) [0x411c3d]
[ 41369.118] 5: /usr/lib/xorg/modules/input/evdev_drv.so (0x40e000+0x524f) [0x41324f]
[ 41369.118] 6: /usr/bin/X (0xcb6000+0x78381) [0xd2e381]
[ 41369.118] 7: /usr/bin/X (0xcb6000+0x9fce8) [0xd55ce8]
[ 41369.118] 8: (vdso) (__kernel_sigreturn+0x0) [0x1cf400]
[ 41369.118] 9: /usr/bin/X (0xcb6000+0x18cad0) [0xe42ad0]
[ 41369.118] 10: (vdso) (__kernel_sigreturn+0x0) [0x1cf400]
[ 41369.118] 11: (vdso) (__kernel_vsyscall+0x2) [0x1cf416]
[ 41369.118] 12: /lib/i386-linux-gnu/libc.so.6 (ioctl+0x19) [0x85a949]
[ 41369.118] 13: /usr/lib/i386-linux-gnu/libdrm.so.2 (drmIoctl+0x34) [0x2b79a4]
[ 41369.118] 14: /usr/lib/i386-linux-gnu/libdrm.so.2 (drmCommandNone+0x31) [0x2ba431]
[ 41369.119] 15: /usr/lib/xorg/modules/drivers/radeon_drv.so (0x47e000+0xa705e) [0x52505e]
[ 41369.119] 16: /usr/lib/xorg/modules/libexa.so (0x3f5000+0x51b8) [0x3fa1b8]
[ 41369.119] 17: /usr/lib/xorg/modules/libexa.so (0x3f5000+0x5614) [0x3fa614]
[ 41369.119] 18: /usr/lib/xorg/modules/libexa.so (0x3f5000+0x56c8) [0x3fa6c8]
[ 41369.119] 19: /usr/lib/xorg/modules/libexa.so (exaMoveOutPixmap+0x4c) [0x3f8e2c]
[ 41369.119] 20: /usr/lib/xorg/modules/libexa.so (0x3f5000+0x5511) [0x3fa511]
[ 41369.119] 21: /usr/lib/xorg/modules/libexa.so (0x3f5000+0xca1e) [0x401a1e]
[ 41369.119] 22: /usr/lib/xorg/modules/libexa.so (exaOffscreenAlloc+0x20c) [0x401c4c]
[ 41369.119] 23: /usr/lib/xorg/modules/libexa.so (0x3f5000+0x5a23) [0x3faa23]
[ 41369.119] 24: /usr/lib/xorg/modules/libexa.so (0x3f5000+0x61b2) [0x3fb1b2]
[ 41369.119] 25: /usr/lib/xorg/modules/libexa.so (0x3f5000+0x3d1f) [0x3f8d1f]
[ 41369.119] 26: /usr/lib/xorg/modules/libexa.so (0x3f5000+0xe4a1) [0x4034a1]
[ 41369.119] 27: /usr/lib/xorg/modules/libexa.so (0x3f5000+0xf092) [0x404092]
[ 41369.119] 28: /usr/bin/X (0xcb6000+0x10d40b) [0xdc340b]
[ 41369.119] 29: /usr/bin/X (CompositePicture+0x21e) [0xdb5efe]
[ 41369.119] 30: /usr/bin/X (0xcb6000+0x10538c) [0xdbb38c]
[ 41369.119] 31: /usr/bin/X (0xcb6000+0x1007f1) [0xdb67f1]
[ 41369.119] 32: /usr/bin/X (0xcb6000+0x3791d) [0xced91d]
[ 41369.119] 33: /usr/bin/X (0xcb6000+0x2535a) [0xcdb35a]
[ 41369.119] 34: /lib/i386-linux-gnu/libc.so.6 (__libc_start_main+0xf3) [0x78c4d3]
[ 41369.119] 35: /usr/bin/X (0xcb6000+0x25699) [0xcdb699]
[ 41369.119] [mi] These backtraces from mieqEnqueue may point to a culprit higher up the stack.
[ 41369.119] [mi] mieq is *NOT* the cause.  It is a victim.
...

These backtrace dumps repeated until the system was forcibly rebooted using the kernel R-E-I-S-U-B magic key sequence. The X server ignored signals HUP (1), TERM (15) and KILL (9).

How shall we proceed?