OK, it took a while but I got the GPU hang finally with kernel3.5.0-27-generic #45~lp1140716v1 :
[22344.085044] [drm:i915_hangcheck_hung] *ERROR* Hangcheck timer elapsed... GPU hung
[22344.085051] [drm] capturing error event; look for more information in /debug/dri/0/i915_error_state
[22344.090106] [drm] Enabling RC6 states: RC6 on, RC6p off, RC6pp off
OK, it took a while but I got the GPU hang finally with kernel3. 5.0-27- generic #45~lp1140716v1 :
[22344.085044] [drm:i915_ hangcheck_ hung] *ERROR* Hangcheck timer elapsed... GPU hung dri/0/i915_ error_state
[22344.085051] [drm] capturing error event; look for more information in /debug/
[22344.090106] [drm] Enabling RC6 states: RC6 on, RC6p off, RC6pp off