hey guys, i realize this post is quite old. but i started having the same issue awhile back. and after scouring the web for what felt like months and trying every supposed remedy, i think i have finally found the fix (that worked for me anyway ).
its easy and is also under the support ticket page on the riot website. simply launch the LoL client, while the client is still running go into ur league folder and delete ur config file (keep in mind all ur setting and key binds will delete so maybe write them down or screen shot) once that's deleted launch a single custom game, after it loads exit out, close out the client and reload the game. obviously you'll have to reset all ur settings again.... but its a small price to pay.
i have tried every fix out there and nothing worked. my pc would freeze basically every game right around the 18- 22 minute mark and i would have to restart my whole machine. since i tried this fix im currently 12 straight games with zero freeze. hopefully this helps anyone still struggling, cuz its super frustrating. ill update post if i see any changes... good luck
also just an fyi, i am currently running windows 10 , not an older version. and i play full screen, non of that windowed mode crap ;)