Created attachment 115201 [details]
/sys/class/drm/card0/error

Using ubuntu 14.04 my i7 Sandy Bridge system with onboard graphics had no gpu problems at all, using kernel boot parameters "i915.i915_enable_rc6=0" and  "i915.semaphores=0".
After upgrading to 15.04 beta it would frequently slow down due to frequent frequent gpu hangs / slow-downs / crashes, with or without the old i915 boot parameters. The system became unusable. 
After a complete re-install using ubuntu 14.10 it crashed once after running stably nearly a day (no i915 boot parameters were given). 
Memory checks had never failed.

This is what dmesg says:

[29907.376682] [drm] GPU HANG: ecode -1:0x00000000, reason: Kicking stuck semaphore on render ring, action: continue
[29907.376684] [drm] GPU hangs can indicate a bug anywhere in the entire gfx stack, including userspace.
[29907.376685] [drm] Please file a _new_ bug report on bugs.freedesktop.org against DRI -> DRM/Intel
[29907.376685] [drm] drm/i915 developers can then reassign to the right component if it's not a kernel issue.
[29907.376686] [drm] The gpu crash dump is required to analyze gpu hangs, so please always attach it.
[29907.376687] [drm] GPU crash dump saved to /sys/class/drm/card0/error

So I did as ordered.