Frequent website security certificate errors - susi.comcast.net redirect to "Activation" page

Been round and round with support and can't seem to get someone who understands the issues.  If I get told to "restart" something again I've had it... and then once I restart something I'm told "you're online so we resolved the issue!"  No.  No you have not and no one seems to have any clue of what is going on.

PLEASE HELP!!!

What happens is when I visit websites, randomly and frequently I get a warning that "This Connection is Not Private" stating there is an invalid security certificate.  The security certificate goes to "susi.comcast.net" and when you proceed to the site it is a redirect to a "Welcome to Xfinity" site telling you to download the app and activate internet.   The websites are where I get this error are random and sometimes it goes through, sometimes it does not. And it's not the same websites either, that is random too.  Many of the websites are standard secure sites that usually have no issues.

I have a personal router, so the Comcast Modem is in bridge mode.  However I have tried several times taking it in and out of bridge mode and still have the same issues either way.

To answer the "tech support" standard questions that I always get asked:

1.  Yes, my internet is activated

2.  Yes, I am connected 

3.  No, I am not "restarting" unless you have an idea of what the problem actually is