Constant crashing

Ever since a few days ago when an alert appeared on the client, something to the effect of "We know the client is occasionally suffering crashes, please repair your client" etc etc, I occasionally just have my whole computer freeze while playing League (and ONLY League). Repairing the client fixes it, but only for that session; if I don't repair, it'll crash about every two minutes or so. If I do repair, it'll not crash at all... and then next time I play, it'll crash about every two minutes or so. Always taking my whole computer down. If I get leaverbustered for this, I'm going to be extremely cross, and I'm sick and tired of being all but unable to play because of it.

Am on Windows 8.1, and this literally never happened before now.