I'm checking the game I'm playing forum to see if others are yelping.
But the 15 minutes to me sounds like heat. Did you pull off the cover and point a fan at the open case for a test run?
my graphics card is very powerful, and can run any game at max settings. However, one time I boot up assassins creed brotherhood, (which normally runs at over 100fps) and I was getting 10-15 fps. I restarted my computer, and then it was temporarily back to normal. I played for about 15 minutes until the fps became horrible again. Other games also have the same problem. Turning down the settings I got the same results. I don't know what might be the problem. I did install a new GeForce driver earlier to my 980ti, and that might be the problem, but I don't know for certain. Can someone please help me?

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