For the past month, Xfinity service slows down to a crawl in the evening

For about the past month, intermittently, and usually after 7pm until midnight (but sometimes till 1am), internet service will be incredibly slow. Slow as in loading google will take upwards of 15 seconds. My pings are through the roof (1300ms to ping google and 700ms to ping devices on our own network). 

I am using an Asus r66u-variant router. 

 

Sometimes the speed drops are intermittent and the speeds will recover for about 1-2 minutes at a time, before dropping again. Other times, it is just a steady slow speed. 

 

When this slow down occurs, we are unable to play any games due to high latency, stream anything in HD (or sometimes at all), and essentially cannot use the internet. 

 

Webpages that don't have any signifcant data content will still load, but do so slowly.

 

What could this be? I tried using a VPN to see if traffic was being throttled like it was several years ago with Netflix, but that did not cure anything. I tried connecting via wireless and via ethernet (to the router) with no signficant change in performance. 

 

Comcast tech support initially said everything is fine, then said there "is a line issue," and then again confirmed no issues. I've gotten a new modem, but am now hesitent to spend money for a new router. 

 

When the pings are high, the pings are also high to other devices on the network, not just the outside internet. For that reason, I'm thinking maybe it's something to do with a failing router, but the router seems to work fine otherwise (in fact, it is fine now). For the past week it's been fine.