Telus gs3 jellybean screen blackout issue

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http://forums.androidcentral.com – I updated to jellybean now whenever I'm watching videos on YouTube or on the browser have no tried my actual videos on phone the screen blacks out during the video. Before it would not blackout unless the video was over and now it black out during which is super annoying is there a way to fix this? Edit: this only happens when watching videos on the browser the screen will black out after the timer is up and it never used to do this before any way I can fix this? (HowTos)