Drop frames on minion and champ interactions

I don't have any screenshots but my PC build is pretty solid. Normally I can run league at 350-400 FPS at full graphic settings. But when a minion comes near my champ, or i'm attacking another champ, my frames drop to 10-12 and sometimes as low as 3 seconds causing a massive lag spike thus giving the other team an advantage. I have tried turning off each graphic setting one by one to see if one of the settings was causing the problem, but no luck. Is anyone else experiencing this and if so have they found out why? It's annoying and I've looked everywhere for a solution.