Mac Pro Keeps Crashing

(See specs below)

I have a Mac Pro from Early 2009 that keeps crashing at random times, but it seems to crash more often when watching a YouTube video.
When it crashes, the mac freezes for about 2 seconds and then turns off. There's never a beach ball and never a shut down cycle. Then it restarts.
I took it into Apple and they accessed it. They said that everything is fine, but I should reformat all four HDDs, which I did. Then two days later, while watching a YouTube video, my computer crashed again.

So, is this a HDD issue? or is this a graphics card issue? or possibly OSX?

Mac Pro
Early 2009

Processor: 2 x 2.93 GHz Quad-Core Intel Xeon
Memory: 32 GB 1066 MHz DDR3 ECC
Graphics: 2 x NVIDIA GeForce GT 120 512 MB
Software: OS X 10.8.4 (12E55)


Hardware Overview:

Model Name: Mac Pro
Model Identifier: MacPro4,1
Processor Name: Quad-Core Intel Xeon
Processor Speed: 2.93 GHz
Number of Processors: 2
Total Number of Cores: 8
L2 Cache (per Core): 256 KB
L3 Cache (per Processor): 8 MB
Memory: 32 GB
Processor Interconnect Speed: 6.4 GT/s
Boot ROM Version: MP41.0081.B07
SMC Version (system): 1.39f5
SMC Version (processor tray): 1.39f5

You should try running the Apple Hardware Test to see if the problem is hardware like RAM or graphics card.
https://support.apple.com/kb/ht1509

Also see my signature for a post detailing how to test your hard drives.

Agree with louishen think you have a gfx problem. Cleaned out the Mac Pro to remove dust etc? Also try removing the gfx card, reseat firmly, reconnect and see what happens.