26.5b5:Blend Torture Test Hard Freeze on 7th Hour

I experienced a hard screen lock while stress testing my machine over night. The image to the screen was frozen. Mouse didn't move. System was unresponsive. Windows System Log for the freeze time was empty with no Errors or Critical Notices. I would like some comments please on what could have been the cause. Also, I would appreciate advice on how to resolve it so it doesn't happen on the next stress test.

*Hardware*
Intel Sandy Bridge i5-2500K
Corsair H50 cooler with Noctua NT-H1 Thermal Compound
MSI P67A-GD65 (B3) Latest BIOS 1.8
G.SKILL Ripjaw (4x2GB) DD3 1600 (F3-12800CL9D-4GBRL x 2)
MSI N580GTX Twin Frozr II OC (1.025 core voltage)
Turtle Beach Montege DDL PCI Sounds Card
GW-USNano USB WiFi-N device
Crucial C300 128GB SSD (o/s) - Attached to Sata III - AHCI
Seagate ST310000528AS 1TB HDD (page file, data) - Attached to Sata II - AHCI
Corsair CMPSU-850TX
Cooler Master CM 690 II Advanced Case with seven case fans

*Software*
Windows 7 Home Premium x64 SP1 Updated to 3/15/2011
- High Performance Power Plan, Turn off HD Never, USB Selective Suspend Setting Enabled, PCI Express LSPM Off
- Page file 2GB set on HDD, Hibernation and System Recovery Off
- Microsoft Security Essentials with Scheduled and Realtime Protection disabled
- DirectX June 2010 Redist
- Screen saver disabled, Turn off Display Never, Sleep Never
Intel Management Engine Components 7.0.0.1144
Intel Rapid Storage Technology 10.1.2.1004 with LPM disable registry fix
NVIDIA Geforce 266.58 WHQL
Real Temp 3.67 ( shows multiplier go from 17.0 to 41.5 [why 41.5?] )
Prime95 x64 26.5b5
MSI Afterburner 2.1.0
MSI Kombustor 2.0.0 (Stress tests GPU in windowed-mode, I selected OpenGL 3.3, 1600x900 window, 16xAA)

*Stress test*
Prime95 Torture Test All Four Cores in Blend Mode ("tests some of everything, lots of RAM tested")
MSI Kombustor Burn In Test Running 3D API GL3, 1600x900 Windows, 16xAA
RealTemp Running
MSI Afterburner Running

!!!!!!!!!*Stress test results*!!!!!!!!!!!!
Hard freeze system lock at 07:00:02 time stress testing.
RealTemp showed lowest core temperature at 32C, highest core temperature at 64C, average core temperature 56C on load
Afterburner showed GPU full load temp steady at 60C
Prime95 - All cores appear to have reached seventh hour of testing before the freeze. The last thing I see on the screen is:
"Self-Test 2800K Passed" and then about two minutes after that, the freeze seemed to have occurred while testing next set.

*BIOS Settings*
CPU Base Frequency (10KHz) 10000
CPU Ratio 33
Adjust CPU Ratio in O/S Enabled
Intel PLL Overvoltage Auto
EIST Enabled
Intel Turbo Boost Enabled
DRAM Frequency Auto
XMP Enabled
Adjust DRAM Frequency 1600MHz
Spread Spectrum Enabled
VDroop Control HighV vDroop
CPU Core Voltage 1.2v
I/O Voltage Auto
DRAM Voltage Auto
System Agent Voltage Auto
CPU PLL Voltage Auto
Active Processor Cores ALL
Limit CPUID Max Disabled
Execute Disable Bit Enabled
Intel Virtualization Technology Enabled
Power Technology Custom
EUP2013 Enabled
CPU Phase Control SVID
C1E Disabled
Intel C-State Enabled
Overspeed Protection Enabled
C-State Limit 70
Long Duration Power Limit 95W
Long Duration Maintained 1000ms
Short Duration Power Limit 118W
Primary Plane Turbo Power Limit 0
Ratio Limit 1-Core 37
Ratio Limit 2-Core 36
Ratio Limit 3-Core 35
Ratio Limit 4-Core 34