I've had this problem quite a lot too, but for much longer.
I tried 2 things to solve it, both worked reasonably for me.
-
Is go to your graphics card settings and change the 3D physics settings to "let the application decide"
-
Is set the affinity of the game from task manager. Unfortunately this one is not a long term fix because you have to change the affinity every time you load the application.
There will be different methods for setting affinity for different OS's, but I do know the Windows 7 way and can tell you.
Open up task manager
Locate leagueoflegends.exe and right click on it
select "set affinity"
uncheck all the boxes and only select 2 or 3.
What this does is forces your league client to only use 2 or 3 of your cpu's instead of the multitude you have. It makes processing data for the application more fluid and removes the jitters and frame rate drops you might experience during your game.
It does not harm your computer, so if it doesn't work for you, don't worry, its not permanent it will revert to normal when you close the application.
Hopefully something will work for you!