Computer shutdown after graphical load

[afflack]

Recently I have been playing S.T.A.L.K.E.R: Call of Pripyat and I've been having some trouble with my computer. After about 20-30 minutes of playing my computer will turn off instantly and I'm a bit puzzled since I know some about computers. I'm concerned that it is my power supply slowly dieing, but I want second opinions before I purchase any upgrades.

My specs:

Windows 7 64bit
Q6600 2.4ghz stock ( I know, bottleneck to the max :| )
XFX 5870 1GB
4GB DDR2 800Mhz RAM
800W BFG Tech Power Supply


I've had the power supply for about 2-3 years now and I don't know if it's performing as it should.

I monitor my temperatures while playing STALKER using HWMonitor and up to 1 minute before my computer turns off I have steady temperatures of 60-70C on the graphics card and 50-59C on the CPU (all cores).

Let me know if any of you need additional information, thanks!

 

[enzo matrix]

I gotta ask how you could possibly think a 2.4GHz Q6600 is causing a bottleneck?

 

[afflack]

I've heard that 3.0 - 3.2 ghz is a recommended amount for a 5870 to perform at its highest potential.

 

[afflack]

Can anyone help me?

 

[has22fas]

Does it turn off with a blue screen?

 

[afflack]

No, it's almost like the the switch on the back of the power supply is turned off, all power to the computer is killed instantly.