[CRASH] Mid-game crash in every second game i play, need to hard reset pc to shut down client.

Every time i play, the first game is fine. The second game however is also fine up until i reach the 20minute mark (2 times i wrote down the actual time and both were 20min34sec ), suddenly the game freezes. Theres stil background audio but not of champions and whatnot, just the regular music and winds and whatever. I cant open task manager to close the client, i cant go to the desktop, i can alt+tab but the window i select wont open. I've ran the repair tool and it stil occurs. My PC is an up to date fairly new rig with components far enough to run the game at highest graphs.