Asus p8z68 windows 7 shutting down unexpectedly

I am having strange issues with a P8Z68 board under Windows 7 64 bits.
I have 3 Seagate ST2000DL003 (CC33) 2TB drives, 2 of which are set as RAID 1 on 2 Z68 3GBs SATA ports.
I do have installed the latest Intel Rapid Storage drivers.
What it happens consistently, and only when in Windows 7, is that under heavy disk-write load, the system
would just silently shut down without leaving any trace of the "why" in the event log except that at reboot
there will be an error in the system log saying that the system did shut down unexpectedly (thanks M$, it really helps).
I have DFT'd the RAID-1 volume and the standalone disk, using 8 parallel instances of diskbench doing batch file write for hours.
Randomly, after a few hours of tests the system would just shut-down and poweroff by itself.
More recently when copying very files (total 750GB) from the RAID volume to the non-RAID one, got same behavior.
On the other hand, I have left the system running for days in Knoppix doing intensive, parellel, R/W tests with badblocks and did never shutdown.
What could be the issue - I am not sure I can RMA the disks or the mobo because otherwise everything seems to point to some Windows auto-protect feature.