PC Restarts during gaming

[Voxx101]

When i'm playing a more resource heavy game such as the Witcher 2, Mass Effect ect. my computer will restart itself after about an hour of play, this is a recent issue, i used to play these games for quite some time with no problems. I am able to play games such as League of Legends, Starcraft 2 and Diablo 3 on high settings for hours on end. After the computer restarts i check the cpu temp using core temp, with temps around the mid 40s (using the stock fan).
Any suggestions on how to fix this?

CPU: i7-2600
Motherboard: Asus P8P67 Pro
RAM: G-Skill Ripjaw 16GB
GPU: Radeon HD 6870
PSU: Antec High Current Gamer series 750W
HDD: Western Digital Caviar Black 1TB
Case: Cooler Master HAF 932 Full Tower

 

[Voxx101]

To be more specific GPU is an ASUS Radeon HD 6870

 

[fantastik250]

What is the temp for the GPU? not only the CPU.

Also have you done any overclocking?

 

[Voxx101]

I haven't done any overclocking

I'll check the the temp on the GPU

 

[Voxx101]

According to Speedfan my GPU is idling around 50C

Just added a Cooler Master Hyper 212+, CPU idles around 32