Connection dropouts every ~15 mins.

Since early October my connection has been dropping out intermittently. Specifically, for at least a month, every 10-20 minutes my bandwidth drops down to anywhere from 0 to 1000Kbps and remains there for up to a minute before returning to normal. My bandwidth is normally ~400 down/12 up.

It happens on all my devices on both ethernet and wifi, and I have no issues when using tethered cell data instead of my xfinity connection. 

It seems to be my upload speed that consistently drops to close to zero, but I also have occasional issues with download speed too. I'm regularly experiencing issues with video calls, online games, and streaming due to this. 

I've had two tech visits in the past couple of weeks. The first visit, the tech said there was a clear signal issue and that a network specialist would be called out to fix it. After the maintenance was completed, the issue still remained and the tech came back out. He checked the line both inside and out and confirmed that the original maintenance had been finished. He then replaced my modem/router. Unfortunately, none of this solved the problem. He acknowledged the issue was still present but said it would be hard to fix and couldn't provide a specific reason for it. 

I'm at a loss now. I've never had an issue like this before and I'll be forced to switch to another provider if it can't be resolved.

I figured I'd post the usual info here just incase, but seen as a tech looked at this just a few days ago, I'm not sure there's anything that'll stand out. Regardless, I appreciate any help or advice on what to do.

What the dropouts look like:

https://imgur.com/a/u0FaDaH 

Router/modem : CGM4331COM (new as of 3 days ago)

The only logs that show up other than logins/initial password change are these from firewall section: 

Signal info