I've seen the same problem posted on here 2 years ago (!) and no riot employee answered it, so I just want to inform you that it is back.
Basically every match ( doesn't matter if it's normal, poro king or even clash ( !!! ) ) crashes around 5 to 15 minutes during the match.
How it happens: I play normally, there is no sign of lag or anything, but then there's a sudden freeze, my mouse cursor turns into "loading circle" and I hear that "boink" sounds which indicates a crash. Then I am unable to do anything, my whole PC freezes, can't even alt-tab or alt-f4, the only solution is to kill the process through task manager after doing ctrl-alt-del.
Normally I can return to the game in around 2 minutes If I react quickly, but it should not be ignored. This is really game-breaking bug. Yesterday it happened twice during my clash ( that's not all of the crashes I had yesterday ) both during combat resulting in my death and enemy getting few minutes of free time on botlane.