FPS drop then game freeze.

I have been starting to get this problem randomly in league games, normals, ranked, customs on every map. What happens is that my FPS is stable at around what I usually get, which is 60FPS, but all of a sudden my FPS drop s to about 20, then keeps going lower, after a couple of seconds it goes to 00FPS and my game freezes, the sound on my computer sounds like it is giving feedback, and pretty much my whole computer freezes, since i try switching tabs and it does not work, neither does Windows button. I have also noticed that this occurs around 10-12min into the game most of the time.

After the game starts again I have about 05FPS for the rest of the game and it is horrible. Can someone please help me, I am losing too many games because of this and it is making me very frustrated.