https://bugs.freedesktop.org/show_bug.cgi?id=94933
--- Comment #5 from Michel Dänzer michel@daenzer.net --- The X desktop size changes from 1280x800 to 3200x1080 around the time the GPU hangs. Can you also reproduce the problem by only changing the desktop size, without changing anything about the external monitor, e.g. using something like
xrandr --fb 3200x1080
?