In-game Heavy Frame drops.

During in the game from the start its perfectly fine but sometimes in mid or late game I start getting frame drops. The frames drop down to 5-16 frames and I don't understand why, I have no problem with playing ANY other game but only league my game starts to drop frames. For some reason it only happens to only certain League games. Champions don't matter it turns out, as I previously thought it was just a Yasuo bug but it seems to be on everyone I play.