Persistent input lag is causing me to lose games

Every game at around 7 minutes into the match, I'm hit with a never ending spasm of input lag. It will delay my actions for 3 seconds. I keep losing team fights and games because it will just start and stop. I've looked up numerous discussions and the only solution is "Did you try Avast?" which I don't have installed. Is there an actual fix or is Riot just not doing anything about this? Why would I want to play a game I know is going to mess up everytime?