You may have sleep mode enabled in it. Also if you are using a gamepad instead of a keyboard, it may not be set to "wake" the computer or be recognized as the computer still in use so it will stay out of sleep mode. You can test this by tapping a key, like CTRL every 5 minutes or so, and if that won't do it, then wiggle the mouse. If one or both of those keep it awake, then you know what the problem is.
My step-dad who is a professional computer tech recently rebuilt my computer for me so that I could experience better game play. Ever since the upgrade I have been having issues. Whenever I play a PC game (the game is Sims 3 but I don't think that matters) my computer will freeze and then the screen goes black and says "Entering Power Saving Mode" I'm not able to wake it back up by moving the mouse or doing a ctrl+alt+del. I have to restart my computer by the power button. This usually happens between 10-30 minutes of game play. It happens EVERY time. The cables are all new, as well as everything in the computer, so I don't believe it's a faulty power source or overheating. I have tried going into my computer setting and changing the power saving and standby modes to NEVER as well as any screensavers, but it still does it. The same thing happens on My step-dad's computer with my game and some of his. We both have the exact same computers and neither of us can figure out what is wrong.
The monitor is a Dell S2240M
It's Windows 7 Professional
The processor is AMD FX 8320 (Eight Core)
64-bit
The Graphics card is an NVIDIA GeForce GT 610

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