Created attachment 116035 [details]
/sys/class/drm/card0/error

I don't know what triggers this, so I'm not sure if there is an existing bug open for this particular issue.  Occasionally the screen will go black and syslog has:

[542758.816056] [drm] stuck on render ring
[542758.816062] [drm] GPU crash dump saved to /sys/class/drm/card0/error
[542758.816064] [drm] GPU hangs can indicate a bug anywhere in the entire gfx stack, including userspace.
[542758.816065] [drm] Please file a _new_ bug report on bugs.freedesktop.org against DRI -> DRM/Intel
[542758.816067] [drm] drm/i915 developers can then reassign to the right component if it's not a kernel issue.
[542758.816068] [drm] The gpu crash dump is required to analyze gpu hangs, so please always attach it.
[542758.817172] [drm:i915_set_reset_status] *ERROR* render ring hung inside bo (0x917b000 ctx 0) at 0x917be0c
[542758.940092] [drm] GMBUS [i915 gmbus vga] timed out, falling back to bit banging on pin 2
[542759.324064] [drm:i915_reset] *ERROR* Failed to reset chip.

Rebooting the machine clears the GPU problem.

GPU:
00:02.0 VGA compatible controller: Intel Corporation Mobile GM965/GL960 Integrated Graphics Controller (primary) (rev 0c)