Mordekaiser FPS drops [Gameplay]

On only my frames drop by about 100. Somewhere around 15 mins my frames drop from 130-140 to 70-80 and then a few minutes later I'm down to 45-50 frames. I have literally no idea why this happens. I can sort of understand some fps drops if your using w or you have a dragon following you around, but this is just me sitting in base down 100 fps. Also to clarify, I run the game on the lowest settings with shadows on low.

I'm not sure if this happens for other people, so I'm not sure if this could be reproduced by other people. The way I can reproduce it would be to just simply play the champion. It has happened to me all 4 times I've played him on the Rift and Howling Abyss.