Anyone else's nMP shutting off/powering off randomly? This happens to me about 2-3 times a week. Usually while the computer is asleep.
1) Are you plugged into a UPS system ?
2) Is your electrical power being disrupted in some manner... ? ...such as voltage drop, frequency variation... check with your electrical supplier about this.
3) Is the MacPro6,1 anywhere near a heat source that may cause the ambient temps to get higher than what the MacPro 6,1 is designed for ?
4) Check your system.log messages for the time you estimate the powering off situation arose. Should be easy to find the messages just before the BOOT message when you discovered the MacPro 6,1 was powered off. Post back the messages as best you can.
5) Were you running any type of heavy workload during the time the power-off situation arose ?
6) Given this issue arises when the MacPro 6,1 is asleep, try leaving the MacPro 6,1 awake to see if this resolves the issue.
FYI... I've had my MP6,1 12-core/D700s/1TB-SSD/64GB-RAM running constantly since Feb 24 and it has never powered off by itself. I do not sleep the MP6,1 either. My MP6,1 is plugged into a UPS system... so even if the incoming electrical power shuts off, the UPS would kick in... but that's only a temporary fix to allow for a graceful system shutdown.