XFX 780i SLI - CPU Overheating - Machine Reboot

Hi, I am sorry to ask a silly question; however, I am going to... :-)

I've recently bought a new motherboard, the XFX 780i SLI so that I can use two EVGA 8800 GTS cards in SLI. OK, I have just re-built my PC (in a new case - large enought to accomodate two cards) with this new motherboard. Since building it I have experienced numerous PC sporadic reboots. I have manage to track this down to the CPU overheating (I think). In the BIOS, after the motherboard POSTS (I think that is the correct term), I watch the CPU temp. I rises fairly rapidly until it reaches 99/100 degrees C. Once it does the machine switches off. This has only happened since I built the machine with this motherboard. I've touched the bottom of the heatsink and it appears to be surprisingly cold, whilst the CPU is blisteringly hot. IS anyone able to help me out on this one?

Many Thanks,

PS. My XFX Ticket Details:


Machine Spec.

CPU: Intel Core 2 Duo - I THINK ITS A 6700 SERIES
Motherboard: XFX 780i SLI
PSU: HX 620 W CORSAIR PSU
Memory: CRUCIAL 2GB DDR2 PC2-5300U-555-12-E0 240-PIN UNBU
Graphics: At time of testing 1*EVGA 8800 GTS
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9/5/2009 POWER ON MACHINE; MOTHERBOARD BEEPS ONCE; DISPLAYS `FF` IN DISPLAY; SWITCHES OFF AFTER AN UNPREDICATBLE AMOUNT OF TIME (USUALLY WITHIN 30 SECONDS OF BEING POWERED UP). THIS HAS HAPPENED SINCE I BOUGHT AN 780I SLI MOTHERBOARD FROM XFX. PLEASE HELP. THANK YOU. PS. ALL OF THE COMPONENTS WORKED PERFECTLY FINE ON MY PREVIOUS MOTHERBOARD, MSI N1996. KIND REGARDS.

[ 9/5/2009] I`ve dug a bit more and discovered that it is my CPU that is overheating since it has been applied to this motherboard. Looking in the BIOS showed me what I needed to see. The CPU temp would climb to 100 degrees C and then the PC would shutdown immediately. This explains why it takes varying amounts of time to reboot after a POST - due to the varying temperatures and verying points in time. Can you suggest why this is happening with this motherboard, and not with my previous one. The heatsink and fan appear to be secured properly. Regards.