your connection is not private problem

a few times per day, I experience an error on Chrome (across multiple Mac laptops) that says “this site can’t be reached — err_connection_refused” and lists the certificate (which is always listed as the same certificate that has Comcast in the url) as invalid, even tho it’s marked as trusted.

Then the website will redirect and say “your connection is not private”

Then usually after 5 minutes, but sometimes up to an hour, the website will resume working. 

I have chatted with Comcast six times about this, and they’ve done a bunch of stuff to my modem: restarted, reconfigured, hard reset, sent stabilizing signals… we even turned off the hotspot feature on our account (as suggested in another thread). But every day the problem comes back. 

I am losing my mind and truly don’t know enough about how wifi works to even know what could be causing this. Why is it only with chrome (other browsers and programs work)? Why is it saying the certificate is Comcast - on countless non-Comcast webpages? Is there something wrong with my modem?