Internet cuts out only during online gaming

I've been having this issue with my internet for about a month now. It runs fine for the most part. I can check email, watch videos, and work from home uninterrupted. But when I log in to an online game I'm disconnected after a few minutes and my computer loses internet for several seconds. Usually it's almost exactly two minutes after I log in, but that number has stopped being consistent over the past two days. Some days it works just fine, but I've yet to find a pattern as to why. So I'm just filled with anxiety as I log in wondering if it's going to work today or not.

The Xfinity troubleshooting tools say my internet is as strong as can be, and when I call the support hotline they sound just as confused as I am as they walk me through resetting my router. I've reset my router dozens of times, usually to no effect. It does seem to work when I'm on call with support, but only for a few hours after I hang up.

Other facts: 

>I'm using a wireless connection roughly 20ft away. I'm planning to buy an ethernet cord and relocate my router to see if that helps.

>All online games are having this issue, though I just use one mostly.

>Games that are not "online" but have online features will stutter and lag every 3 seconds or so as it is apparently trying to do something internet-related in the background.

>I've heard the word "bandwidth" when asking around, but I'm not sure how to check how much bandwidth I have or if I'm exceeding it during these times.