Hm, I just thought something!
My main personal development machine had the same problem some months ago. I have three monitors attached to the PC to help me with PHP development, and I had one of the monitors attached to the motherboard's internal Graphics Processor, the Intel HD Graphics 4000 which is part of the CPU.
This PC shutdown unexpectantly many times, and without giving much thought, I suspected the GPU as the culprit - and it really was. It was not the first time I had stability issues with an Intel GPU chipset! I attached the 3rd monitor to the main GPU (an AMD card) and problems disappeared since.
As I am realising now, only the GPU drivers are so connected with the OS core they can cause a sudden shutdown. So Intel drivers may are the source of the problem!
I cannot check my hypothesis until Monday, but the PC with the recent shutdown problem is also running with an onboard Intel GMA3150 chipset GPU. Maybe the GPU tries to access a RAM memory area the wrong way, so the PC shuts down.
So, I have to ask you: What kind of GPU each of you are using?
If all of you are using an Intel onboard chip, we may have found the culprit!