Dell XPS 720 shutting down unexpectedly

Hello guys - - I'm having a problem recently with my Dell XPS 720 rig that shuts down by itself unexpectedly after 15-20 mins. I can't figure out the problem with it please help! Here's the specs:

Dell XPS 720 Q6600 2.4, 8mb, Kentsfield GO
4GB Kingston HyperX DDR2 PC-6400 800mhz
2TB harddrive
750W PSU
2 Nvidia 8800 GT in SLI
Razer Barracuda soundcard
Windows 7 Ultimate
30" Dell-3007wfp

I don't know if this could be the problem... I bought a 9800GX2, downloaded the driver and tried to install it but after the system reboot, the monitor just went blank. So I restarted the pc, and the option to open the pc to "safe mode" appeared. So I decided not to install the new card and deleted the driver for it. I restored the pc to the last known date, to no avail. It kept on shutting down on me.

I went to the "event viewer" in "my computer/manage" and this code appeared as a "!": WinRT and OMCI...

Please help, I don't know what to do. Thanks in advance.
 
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