Computer freezing only with league

When I play league of legends my computer completely freezes at one point of my playing everyday. This only happens in league of legends and when it does happen nothing works at all, not even alt tab or ctrl alt delete. The computer as a whole becomes completely frozen and needs to be turned off by holding the power button. During this freezing I do not DC from the game until well after my computer's off. This usually gives my lane matchup a free kill. I have downloaded the alpha client and with the alpha this happens even more than with the previous client. I can not play a game on alpha client due to it freezing everytime I get into a game. I would like to know if anyone has an idea what's causing this problem because it is a more recent problem and had not happened previously. Also my computer, no matter the graphics I put it on remains at the same fps it will bounce anywhere from 60 to 9. Before the freezing I could play on max settings with 60 fps without a hiccup. If anyone has an idea what's wrong please let me know and please let me know a potential way to fix this problem I am having. I have tried the repair tool as well