If heat is a problem. I suggest to turn off you PC and wait for it to cool. Then after it has cooled down open it up and disconnect your keyboard, monitor and other stuff. Check to see if there is any lint, dust, hair, etc. around or on your processor, memory, video card, motherboard, and fans. Get a can of compressed air and spray it (preferably outside). Connect everything back up to it but don't close the side of the computer case. Then turn on your PC. Check if your fans are running cool and blowing air onto your processor, video card. If this is working. Then shutdown your PC and put the case back on.
Now, if this is a recent problem then it could be that one of your components are going out or maybe something is awkward about the software on your pc. If this only happens when you play video games then it may be a result of a bad shutdown, outdated drivers, incorrect BIOS settings, and/or the specific 3d setting(s) for you graphics card/integrated graphics. Sometimes it could be the video game itself. There maybe some services packs or updates to your games.
Try reinstalling the video card drivers, or download new drivers if possible. Then reinstall the latest DirectX for your operating system.
Try these for suggestions for now, and let us know if they work. Also if you could be a little more descriptive on what happens or error messages come up, and the name of the games your attempting to play on which operating system.

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