Random Freezing

Good afternoon guys.

I built a computer and I've been having some strange issues. Here's my profile:

OS Version: Microsoft Windows 7 Home Premium 64 bit
System RAM: 8168 MB
CPU Name: Intel(R) Core(TM) i5-2500K CPU @ 3.30GHz
CPU Speeds: 3301
Physical CPUs: 1
Virtual CPUs: 4
Video Card Description: NVIDIA GeForce GTX 560 Ti X 2
VRAM: 1023 MB
Primary Display Resolution: 1920x1080
Multi-Monitor Desktop Resolution: 3600x1080
Microphone: True
Language: English (United States)
Free Hard Drive Space: 11018 MB
Total Hard Drive Space: 953866 MB
Windows Experience Index Rating: 5.9

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I have a Corsair Enthusiast 750 PSU.

My computer freezes repeatedly when I attempt to play certain games. Not all of them. Some games work just fine, others don't, and I can't seem to detect any rhyme or reason to it. Sometimes games with absurd system requirements will run perfectly while another game with lesser requirements might freeze over and over again.

I ran stress and memory tests on my video cards and everything turned up fine.

Here's the kicker: those games that don't work? If I load them as soon as my computer starts up they will play perfectly. Let's use Guild Wars 2 as an example. If I start my computer and then immediately start the game I can play for hours on end without a single hiccup. If I spend 15 minutes or so checking my email, then load the game, every few minutes it will freeze for a few seconds and there will be a slew of graphical distortions.

Or if I start the game right away and then alt-tab out of it for a moment it will start freezing when I return to it.

I'm absolutely baffled. I've heard it might be that my PSU doesn't have enough power. According to my calculations I should have enough juice for everything. I haven't done any overclocking yet. I've repeatedly updated my video card drivers. I'm at a total loss. I'm thinking about testing my video cards individually to see if maybe one of them is faulty. I don't know what else to do.

Any suggestions?