xfinity stream error messages

I've used Xfinity Stream with ROKU on two TVs for at least 3 years and seldom had a problem.  Within the past 2-3 months, Xfinity Stream has become almost impossible to use.  It repeatedly buffers or freezes when I'm trying to watch a TV show...doesn't matter which show.  It usually happens with recorded shows but has started to happen with "live" shows.  I get messages like "Attempting to Resume... We've run into a problem while streaming."  "Sorry! Something went wrong. Please try again later. TVAPP-00800" "This video cannot be played. We've run into a problem while streaming.  TVAPP-00307"  This problem is not related to poor internet service, based on speed tests and the strength showing on my cellphone.  The problem isn't related to heavy internet use while trying to watch TV.  We are retired, don't play Call of Duty and seldom have multiple devices in use at the same time.  I've rebooted the router/modem numerous times, done refreshes, checked cables...no change.  Dealing with your Chat bot is typical of today's climate of "non-service" (the wait to speak to a human at Delta is 4-8 hours) so I went to the nearest Xfinity store a couple of days ago.  The rep immediately decided I had a bad router/modem and gave me a new one.  I installed it, moving it to immediately above the TV that I use most often.  Did that fix the problem?  Of course not.  I've been a Comcast customer for as long as you have been in Indianapolis...40+ years.  This needs to be fixed.  MetroNet just installed new fiberoptic cable in my neighborhood with a junction box at the corner of my yard...and their pricing is attractive.