Early game FPS drop problem

In the beginning of any match my FPS is jumping around between 100-150, this is above my monitors 60 GHz refresh rate so no complaints there. However, 2-5 minutes into the game my FPS will DROP to 30-50 almost never surpassing 60 frames :( I have made sure this is not an overheating problem nor has it anything to do with the amount of sprites (champions or minions) l am able to see in-game. Any suggestions on why this happens and how to fix it?

Specs: Windows 10 Pro Intel Core 2 Quad Q6700, overlocked to 3GHz 8GB DDR2 RAM GeForce GTX 660 graphics card Game is running off of RAID1 hard drives