Black Screens and Framerate Drops

Im not sure if this is a bug or not. It started when I downloaded the game. I start the game and everything is normal, then out of nowhere the framerate drops from 60 to 10 and then when it reverts back to normal, everything that causes light in the game just turns black, including champion abilities. Sometimes, the screen will suddenly turn black and then pop back to the game 10 seconds later. I have tried reinstalling, patching the files, and even updating my drivers but nothing has worked. If anybody could give me any tips or advice on how to fix this problem, I would be overjoyed. I just want to keep playing this amazing game. :)