Same problem started for me months ago. I went through the whole trouble shooting process with Tech support (gotta give them credit for trying). But nothing was fixed. They seem to be insistent that is has to be a problem on my end. Yet I played other online games on the same computer and none of them had any crashes. Tech was thinking maybe it was a conflict with my anti-virus Avast. I even tried turning it off to no avail.
I ended up upgrading my computer anyway a couple of months into the process.
Shock!!! Brand new Asus Scar II gaming laptop ... much better, crashes less, but still crashes and today I had it completely freeze up again just like the old one.
In fact LOL would still crash from time-to-time on a brand new, fresh computer with nothing else installed!
Solution in short (I published a longer version on other threads) is to just UNINSTALL and REINSTALL. But make sure that you put in a formal support ticket to RIOT requesting support so that they know you are having the problem. If you put in a ticket I guarantee someone will get back to you because they always do.
Their techs will give you really good information on trouble shooting and depending on how dedicated you are to following it all up it can help you optimize your system for LOL. But in the end nothing they say will really fix it, it is an underlying problem in the LOL system. The only real solution is a regular fresh install.