PC Shuts down during ARAM and Treeline but not on the Rift

It doesnt freeze or disconnects, it just shuts itself down. And only during Treeline and ARAM. I've done some internet searching and I see that other encounter the same problem in Howling Abyss.

I have lowered the settings but it never had an issue with it, I have a reasonably okay computer, can play on high with 140 FPS but capped it at 60 to be sure, but it doest matter. Lowering settings to medium dont do the trick, but I never experience any problems at all when im on Summonners Rift, i can play with even higher settings and it never fails, bugsplats or anything.

What can I do to fix this? I'd like to play Hexakill too >__>