Game freezes completely around 15-25 minutes

A few days ago every one of my games just started freezing and forcing me to restart my computer, when I get back it's too late because the game is already lost. I'm pretty sure it's not the computers fault because i've been playing in it for almost 3 years and it never had a problem. I also started getting really low fps when 2 or more champs are shown in my screen. All of this started with the new patch. I've tried reinstalling and even used hextech repair tool but it nothing happened.