I recently put together a new computer. Here are the specs:
Intel Core i7-2600k (Stock speeds, Hyper212+ Cooler)
Radeon 6870 X2 in CrossFire (Latest drivers installed a few minutes ago)
16GB DDR3 1600 Corsair RAM
GIGABYTE GA-Z68XP-UD3 Intel motherboard
As far as I know, nothing goes over 40C with general usage. It's not a heat problem by any means.
The problem starts when I start using the GPU with more "intense" tasks. This ranges from watching a YouTube video in 720p, to playing Skyrim.
When doing one of the above tasks, the system randomly reboots; again and again.
So far I've crossed out a few things of what it could be:
Heat (nothing over 40C so far with general usage)
Disk drives (this error occurs in OS X Lion, on my SSD, and Windows 8, on the hard drive)
CrossFire (I took out the other card and have the same issue. The card that's in the system is fine - I've used it in my old computer for hours on end)