An "authentication problem" suggests that you're using the wrong password. It's not at all clear what "Got Xfinity to fix that I believe by reconnecting modem with password" implies, but perhaps the Xfinity tech changed your wifi password.
What do you mean by "I have been having intermittent connection with Xfinity"? Are you referring to connecting by wifi with your laptop or Kindle or do you mean that even when you connect to the Xfinity modem by Ethernet (cable) your connection sometimes drops out?
View your wifi password and write it down. See several methods here: https://www.xfinity.com/support/articles/view-change-wifi-password
Now take a look at the wifi password saved on your laptop: https://www.youtube.com/watch?v=68Q4Km5TIwo
If the password in your laptop is NOT the same as the one you viewed above, change it to match.
While you're at this screen, write down the "Security type" and "Encryption type" should further troubleshooting be necessary.