I have the same exact problem.
Intel 6700 Quad Core
Evga 9800 GTX
Windows 7 64 and 32 bit
Evga 780i motherboard
OCZ 8 gigs of ram
It doesn't matter what OS I am running, Windows 7 32bit or 64 bit, I get the same results. The time that the computer screen goes black and reboots varies but usually after 15 minutes.
I've updated all my drivers and even flashed the bios.
The other weird thing is that this computer has dual boot to Windows XP. I have the same driver versions on both 7 and XP. I can play on XP all day long without an issue.
Its not just SCII that crashes. DAO: Origins does the exact same thing when I play on Windows 7.
There has to be something going on with Windows that is causing the computer to crash because its the same hardware and works like a champ on XP.