Online Gaming - Ping Inconsistency Assistance

Hello,

Since I've started using Xfinity's service in my apartment I've dealt with constant ping inconsistency. I live alone and there is only 2 devices on the network at a time (PC & iPhone). I've tried troubleshooting this issue several times and I have nowhere else to turn to. I can't switch providers because Xfinity is the only provider that services my building. Every time I try to speak to someone I'm told the connection is solid and "there doesn't appear to be an issue". I'm so frustrated with the situation.

I pay for 1gb down and 35mb up. Every speed test I run claims to be in the ballpark of those ranges. The issue begins when I load up any online service games. Call of Duty or Rocket League for example; I usually sit around 40 ping on both games for the first 10 minutes. After about 10 minutes of game-play my ping will start to fluctuate in a 40 point range. I will go as low as 30 ping and as high as 70 ping. This rapid and constant fluctuation makes online service games basically unplayable. All of my movement and decisions are unreliable so it just honestly makes the gaming experience miserable.

It is the most obnoxious, frustrating, and inconsistent experience I've ever had with Xfinity. So, what else can I do? Should I pursue a new rented modem/router? Should I purchase a new modem privately? Is there some magic troubleshooting that can fix this? I am desperate to get this resolved so please, if you have any recommendations/solutions, it would be appreciated.

TL;DR - My ping is fluctuating constantly. How can I fix it?