Strange connectivity over 6 months now

My client has Comcast business at his home. He moved in earlier this year and issue has been occurring shortly after the move.

So over a period of 3 months trying to work with Comcast, we basically gave up. Luckily the problem went away for a bout 2 months. It is now back and the same issue is occurring. Here are the details.

-Called Comcast and they confirmed they can ping the public gateway IP and the one useable IP (our firewall). Because a ping works, the tech thinks everything else will work so he hung up on me when I said this is the issue we have been having for over 6 months now. How can you help?
-Called customer at 5:30AM to troubleshoot one last time. from his PC

-ping 8.8.8.8 from his PC works

-ping google.com fails. It cannot resolve host name

-DNS server is 75.75.75.75
-Can ping XXXX-SERVER (his server at home via hostname. That resolves properly)
-Support shows line signals are good.
-Customer reboots modem to fix issue.

-When this occurs, it is usually anywhere between 1AM - 6AM

-Usually happens X:03AM where X equals 1-6AM

-Usually resolves itself at X:58AM or X:59AM

This is clearly a DNS issue which I confirmed a while ago as well as today. I think port 53 is having issues during this time. Unfortunately, support just knowledgeable on what to do or even how to handle it. Can anyone help?