Posible issue with GPU

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I installed a new GPU about a month ago, and recently have been having issues in game (WOW). I don't know if it's a GPU issue or not.

The screen will flash for a few seconds then stabilize. I'll go from 160 - 200fps in game to 25. I attached a video I took of what happens.

System: 8700k, Gigabyte AORUS 1080 ti Xtreme.

Any ideas?

http://[https://www.dropbox.com/home/WOW?preview=MOV_20180619_1849333.mp4/url]

 

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Ok, so in the WOW settings, if I play the game in Full screen mode I get 165 fps and the screen flashing stops and when I use Windowed (Full screen) mode I get 25 fps. I always preferred Windowed (Full screen) since I can tab out to other things more easily and still keep my main monitor in the game. Not exactly sure why this is happening though.