Secure Connection Failed

Hi - In the past few days I started getting this messages on many legitimate websites.  In particular this happens a lot on xfinity, but also on Vanguard.com and some other sites. 

******************the message I get***********************************

Secure Connection Failed

An error occurred during a connection to www.xfinity.com.

    The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
    Please contact the website owners to inform them of this problem.

********************************************************

This only occurrs when I am using wired connection.  If I change to to wireless then I don't have the problem.  I like the security of a wired connection.

I changed nothing on my configuration.  I have this problem on firefox and edge.  Have gone through clearing history and cache to no avail.  I contacted Dell (my PC support) and they say it is not a problem with my hardware.  They are sure it is a "packet" issue with Comcast.  After hours on the phone with Comcast they are completely incapable of giving any ideas on how to fix this.

Xfinity sent a tech out today who changed my modem but still have the same problem.

HELP!