Crashing a lot

Since about 8.18, both my game and client started crashing frequently. I asked riot support about it and they claimed it to be an issue with my internet. Though I admit my internet back then had some issues every now and then, that would not explain the loadingscreen crashes I had.

A few weeks ago, my internet provider came by to fix some issues and since then everything had been working perfectly - except for league. I haven't crashed in loading screen since last patch, but a new issue had been ruining my games since. About every other game, my client suddenly crashes in the middle of my game, which causes it to pop up some window about the crash, freezing my game. After I decided to enable "close the client during game" option, the game itself would just freeze randomly, with or without closing itself.

At this point I have no clue how to fix it anymore. Hextech-repair tool hasn't helped one bit and it seems like support doesn't know what's causing it either. Any advice?