Kernel: 3.5.0-26-generic #42-Ubuntu SMP Fri Mar 8 23:18:20 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
xorg-server 2:1.13.0-0ubuntu6.1

This is the fourth or fifth time since setting up the machine I have encountered this freeze. When it happens, my display is unresponsive. It seems that applications continue to run in the backend, and possible that I could SSH in (did not have sshd running). I am not sure if I could have killed X. I am unable to switch to any TTY with CTRL-ALT-FKEY; even though the display doesn't update I've tried to blindly login and restart the box but it does not appear to be responding.

Here is the stacktrace from the most recent; I will start tracking these, but I am pretty sure that it is the same as the last one:

[ 36877.815] (EE) 0: /usr/bin/X (xorg_backtrace+0x36) [0x7f74824a9b76]
[ 36877.816] (EE) 1: /usr/bin/X (0x7f7482301000+0x1ac9a9) [0x7f74824ad9a9]
[ 36877.816] (EE) 2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f7481627000+0xfcb0) [0x7f7481636cb0]
[ 36877.817] (EE) 3: /lib/x86_64-linux-gnu/libc.so.6 (0x7f748028b000+0x802ef) [0x7f748030b2ef]
[ 36877.817] (EE) 4: /lib/x86_64-linux-gnu/libc.so.6 (__libc_malloc+0x75) [0x7f748030dfb5]
[ 36877.817] (EE) 5: /usr/lib/x86_64-linux-gnu/libpixman-1.so.0 (0x7f7481195000+0x45e3b) [0x7f74811dae3b]
[ 36877.817] (EE) 6: /usr/lib/x86_64-linux-gnu/libpixman-1.so.0 (0x7f7481195000+0x46e54) [0x7f74811dbe54]
[ 36877.817] (EE) 7: /usr/lib/x86_64-linux-gnu/libpixman-1.so.0 (pixman_region_subtract+0x9c) [0x7f74811dd5dc]
[ 36877.817] (EE) 8: /usr/lib/xorg/modules/libexa.so (0x7f747e2a8000+0x5af6) [0x7f747e2adaf6]
[ 36877.817] (EE) 9: /usr/lib/xorg/modules/libexa.so (0x7f747e2a8000+0x83e0) [0x7f747e2b03e0]
[ 36877.817] (EE) 10: /usr/lib/xorg/modules/libexa.so (0x7f747e2a8000+0x4d3f) [0x7f747e2acd3f]
[ 36877.817] (EE) 11: /usr/bin/X (0x7f7482301000+0x130550) [0x7f7482431550]
[ 36877.817] (EE) 12: /usr/bin/X (ValidateGC+0x24) [0x7f7482369324]
[ 36877.817] (EE) 13: /usr/bin/X (0x7f7482301000+0x12371c) [0x7f748242471c]
[ 36877.817] (EE) 14: /usr/bin/X (miCompositeRects+0x8a) [0x7f74824248ca]
[ 36877.818] (EE) 15: /usr/bin/X (0x7f7482301000+0x12b615) [0x7f748242c615]
[ 36877.818] (EE) 16: /usr/bin/X (0x7f7482301000+0x55a91) [0x7f7482356a91]
[ 36877.818] (EE) 17: /usr/bin/X (0x7f7482301000+0x4456a) [0x7f748234556a]
[ 36877.818] (EE) 18: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xed) [0x7f74802ac76d]
[ 36877.818] (EE) 19: /usr/bin/X (0x7f7482301000+0x448ad) [0x7f74823458ad]