computer freezing

Hi guys so i recently just build my first pc specs can be found here .The issue im having is that it randomly black screens and is annoying when playing games as it kicks me out if you could give me any advice much it would be much appreciate here's a link to a video of what happens...

i've had a pc for a little while now which i had built by overclockers. for some reason it keeps freezing whenever i try and play games, especially after a cold start. this happens with any type of game even if it shouldn't be demanding on the pc. it's also started to freeze if i'm editing video...