Computer Freeze Problem

(excuse bad english) Hey I recently built a new Desktop (it's not the first time I do this). My choice of hardware was mainly made for CGI development. These are my specs: i5 3200, 16Gb RAM, motherboard Asrock B75, socket 1155, 2x side case coolers, 750W PSU, and an Nvidia Geforce GTX 660 2Gb GDDR5. The Desktop works fine in Photoshop, Illustrator, After Effects or Blender and I'm getting pretty good workflow with it. My problem occures when playing games. When I bought my Graphics card it came with an Assasins Creed and Splinter Cell coupon. Both games just freeze after 15 to 20 minutes of gameplay. In AC4 usualy the sound crashes first and after a minute or twoo the Video freezez. I can ALT TAB to Windows but after a minute or so the mouse starts to glitch an eventually freezez the hole sistem. I also tryed Call Of Juarez Demo and it also freezez after 5 minutes or so. Any suggestions? I hope you guys can help me out after building this desktop I kind of ran out of cash and I don't really afford taking my PC in a service because I work on it with a lot of ongoing projects. Thanks![FONT=Helvetica][/FONT]
 
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