Major FPS problem

I was playing my final placement game in flex (Whether i won or lost i was obviously doomed to bronze 5 so i thought might as well get it over with) with garen top. As i was heading towards my turrent my sound had went off completely. Its happened before so i ignored it for the sake of the game and proceeded to play. A Few minutes later and my Fps begins to jump around a bit. It went from about 15-0. It then settled at a steady **01 FPS. ** I had ignored it hoping it would revert to its normal 20-40 (I have a toaster) and instead it kept its constant 1 FPS. I quit the game in hopes that I could reconnect and the FPS would be stable again. I disconnect and the game freezes on the leaving screen. Computer goes black, Crashes completely, and restarts. I log back in as fast as i can only to find myself with 5 Low-Priority queue games and an extremely Laggy client. It takes almost like a minute just for my summoner profile to load. That is if it can handle the random freezes and extreme slowness. After that game my FPS has just been extremely low. Please help