Comment # 3 on bug 1104851 from Dr. Werner Fink

(In reply to Stefan Dirsch from comment #2)
> Still Sandybridge. In case, instead of modesetting driver please also try
> xf86-video-intel driver. You could also try with
> 
>  Option "AccelMethod" "UXA"
> 
> with intel driver.

Thanks a lot!  It seems that in the kernels command line the option
i915.modeset=1 together with Driver="intel" in the old
/etc/X11/xorg.conf.d/99-local.conf do not work together.  After using
Driver="modesetting" and the sepcific option AccelMethod set to "UXA" it seems
to be stable.