Well, its certainly a strange problem. Both the P67 and Z68 I have behave themselves. The third point I suggested is deffinately worth a shot though. Changing the LLC level will give just a touch more vcore to the CPU. One thing I did notice when testing out the P67 and Z68 is that the P67 defaults to 1.18v vcore, while the Z68 defaults to 1.16v which makes me think Asrock may of reduced the potency of their LLC a tad with the Z68 as a lot of people got unexplainable BSODs with the P67 if setting LLC to "Level 1". Best guess is that LLC setting pushed voltages too far out of spec and hence the BSODs. Now it seems like it could be the opposite problem, if LLC potency was reduced a tad because of the BSODs when setting "Level 1" on the P67 board, now the BSODs on the Z68 may be because the "Level 5" LLC setting isn't quite giving enough juice for people using the IGPU.

The simple answer could also always be theres a bug in intels chipset / IGPU driver, but I thought I would include the long glamorus techie theory as well