Internet not stable

Hi,

I just joined Xfinity for half a month, and the experience has not been so good. The internet package for me is 300 Mbps and I used wired internet for my laptop. However, when I am playing online games like League of Legends, there are two or three times the internet jumps from 40ms to almost 400ms which ruined my game experience. 

Could somebody give me some advice to fix this?