Here are some interesting entries from my X log. According to the below page, a likely root cause is GPU lockup.

https://wiki.ubuntu.com/X/Troubleshooting/Freeze#Problem:__Log_shows_.22.5Bmi.5D_EQ_overflowing.22_and_X_freezes

** snip **

[  2995.858] [mi] EQ overflowing. The server is probably stuck in an infinite loop.
[  2995.858] 
Backtrace:
[  2995.936] 0: /usr/bin/X (xorg_backtrace+0x26) [0x4a2656]
[  2995.936] 1: /usr/bin/X (mieqEnqueue+0x1f4) [0x4a1914]
[  2995.936] 2: /usr/bin/X (xf86PostMotionEventM+0x97) [0x47d797]
[  2995.936] 3: /usr/bin/X (xf86PostMotionEvent+0xd0) [0x47d9c0]
[  2995.936] 4: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f13877e1000+0x514a) [0x7f13877e614a]
[  2995.936] 5: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f13877e1000+0x6d66) [0x7f13877e7d66]
[  2995.936] 6: /usr/bin/X (0x400000+0x6c687) [0x46c687]
[  2995.936] 7: /usr/bin/X (0x400000+0x124c1e) [0x524c1e]
[  2995.936] 8: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f138e58d000+0xfc60) [0x7f138e59cc60]
[  2995.936] 9: /usr/lib/xorg/extra-modules/nvidia_drv.so (0x7f1388cba000+0xd9cff) [0x7f1388d93cff]
[  2995.936] 10: /usr/lib/xorg/modules/libwfb.so (wfbBlt+0x124) [0x7f138887f7b4]
[  2995.936] 11: /usr/lib/xorg/modules/libwfb.so (wfbCopyNtoN+0x2c0) [0x7f1388884760]
[  2995.936] 12: /usr/bin/X (miCopyRegion+0x19d) [0x55289d]
[  2995.936] 13: /usr/bin/X (miDoCopy+0x392) [0x552d92]
[  2995.936] 14: /usr/lib/xorg/modules/libwfb.so (wfbCopyArea+0x4c) [0x7f13888849cc]
[  2995.936] 15: /usr/lib/xorg/extra-modules/nvidia_drv.so (0x7f1388cba000+0x45ee89) [0x7f1389118e89]
[  2995.936] 16: /usr/bin/X (0x400000+0xde7d2) [0x4de7d2]
[  2995.936] 17: /usr/bin/X (0x400000+0x2a1f9) [0x42a1f9]
[  2995.936] 18: /usr/bin/X (0x400000+0x2e2a9) [0x42e2a9]
[  2995.936] 19: /usr/bin/X (0x400000+0x21a7e) [0x421a7e]
[  2995.936] 20: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xff) [0x7f138d4d6eff]
[  2995.936] 21: /usr/bin/X (0x400000+0x21629) [0x421629]

** snip **

[ 17465.130] [mi] EQ overflowing. The server is probably stuck in an infinite loop.
[ 17465.130] 
Backtrace:
[ 17465.130] 0: /usr/bin/X (xorg_backtrace+0x26) [0x4a2656]
[ 17465.130] 1: /usr/bin/X (mieqEnqueue+0x1f4) [0x4a1914]
[ 17465.130] 2: /usr/bin/X (xf86PostMotionEventM+0x97) [0x47d797]
[ 17465.130] 3: /usr/lib/xorg/modules/input/evdev_drv.so (0x7f1388204000+0x5ff3) [0x7f1388209ff3]
[ 17465.130] 4: /usr/lib/xorg/modules/input/evdev_drv.so (0x7f1388204000+0x668d) [0x7f138820a68d]
[ 17465.130] 5: /usr/bin/X (0x400000+0x6c687) [0x46c687]
[ 17465.130] 6: /usr/bin/X (0x400000+0x124c1e) [0x524c1e]
[ 17465.130] 7: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f138e58d000+0xfc60) [0x7f138e59cc60]
[ 17465.130] 8: /usr/lib/xorg/extra-modules/nvidia_drv.so (0x7f1388cba000+0xd9cff) [0x7f1388d93cff]
[ 17465.130] 9: /usr/lib/xorg/modules/libwfb.so (wfbBlt+0x124) [0x7f138887f7b4]
[ 17465.130] 10: /usr/lib/xorg/modules/libwfb.so (wfbCopyNtoN+0x2c0) [0x7f1388884760]
[ 17465.130] 11: /usr/bin/X (miCopyRegion+0x19d) [0x55289d]
[ 17465.130] 12: /usr/bin/X (miDoCopy+0x392) [0x552d92]
[ 17465.130] 13: /usr/lib/xorg/modules/libwfb.so (wfbCopyArea+0x4c) [0x7f13888849cc]
[ 17465.130] 14: /usr/lib/xorg/extra-modules/nvidia_drv.so (0x7f1388cba000+0x45ee89) [0x7f1389118e89]
[ 17465.130] 15: /usr/bin/X (0x400000+0xde7d2) [0x4de7d2]
[ 17465.130] 16: /usr/bin/X (0x400000+0x2a1f9) [0x42a1f9]
[ 17465.131] 17: /usr/bin/X (0x400000+0x2e2a9) [0x42e2a9]
[ 17465.131] 18: /usr/bin/X (0x400000+0x21a7e) [0x421a7e]
[ 17465.131] 19: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xff) [0x7f138d4d6eff]
[ 17465.131] 20: /usr/bin/X (0x400000+0x21629) [0x421629]