This is my rig that I set up a couple of days ago. It has shut down during the night two nights running and then hung on restart so I had to dismantle it to rearrange the cards and just to check over ALL of the connections. (There's quite a few.)
I didn't expect it to be easy so it is no big problem. HAPPY DAYZ : )
Could it be a little low on power? what PSUs you have?