My friend with this problem so I posted it here for him.
"I have the downloaded GPU gadget from the internet and have had a couple issues in the past with LoL randomly laaging and dropping to 10 fps when I ALWAYS pull 120+ on ultra (have it capped 60). When it drops to 10 fps, the gadget says its using 100% of GPU (which makes sense).
However, when LoL runs the way its supposed to, it uses only about 30% of my GPU memory on max settings.
They way I normally resolve it is just by exiting the game and "returning to game". I do not have to close the client to get it to return to normal (just reload the game 2-3 times normally).
I also have been having issues with sprites not showing up... things like blitz arm, Amumu bandage toss, gragas barrel (only see it when it explodes).
Just wondering randomly causes these massive frame rate drops and over use of the GPU.
I have a gt 545 with Nvidia driver version 306.97"