Intermittent "err_timed_out" on most websites

Started getting "err_timed_out" about 24 hours ago on all devices - Windows 10 PC connected by ethernet cable, Windows 10 PC by WiFi, Android Smart Phone and Roku Streaming Stick by WiFi.

It will usually last a few minutes and then all websites start working again.  5 , 10, 15 minutes later the errors start again.  While this is happening some websites are not effected, specifically anything related to Google - Youtube, Gmail, Google search, etc...  all continue to work with no problems.  Comcast sites continue to work, although they get sluggish.  Sites like Amazon, Ebay, Bank of America all stop responding and produce the error.

I've tried restarting the comcast modem/router(Cisco DPC3941T) - It didn't help.  Cleared cache - that didn't help.  I've tried some other troubleshooting suggestions from this site:

How to Fix the ERR_CONNECTION_TIMED_OUT Error (Step by Step) (kinsta.com)

But nothing worked - I'm guessing the modem/router needs to be replaced?  Any suggestions would be appreciated.