R6230 predictably drops all ports for 5 secs after 30 mins, 60 mins, 30 mins, 60 mins ...

I have my Nighthawk R6230 downstream from a Netgear CM600 modem connected to Xfinity boradband and regularly at intervals of 30min, 60min, 30min, 60min, 30mins, etc. will drop connections to all attached devices, both wired and wireless for 5 seconds.   Most of the time streaming content and web browsing isn't affected or noticed, but any more sensitive connections, e.g, to my employer over VPN do notice and it's very aggravating.  I have to re-initiate and re-authenticate connections all day long per the above mentioned schedule.

This has happened before for long periods, has miraculously resolved itself for long periods, and is now back once again.

These events are trackable on several devices:

A Meraki cloud managed security device wired to the R6230 becomes unreachable on the same schedule at the same times, according to the cloud.

A Windows workstation wired to the R6230 running a continuous ping to the upstream default gateway (Xfinity next hop router) drops 5 packets on the same schedule at the same times.

A Macbook Air wireless connected to the R6230 running a continuous ping to the upstream default gateway (Xfinity next hop router) drops 5 packets on the same schedule at the same times.

The web based management interface on the CM600 is hardly more than a dashboard, but was indicating that the channel signals weren't very good and the log showed numerous timeouts.  Thinking this must be the culprit, I had Xfinity come out and take a look at it and they said there was in fact a lot of line noise, so they replaced a noise filter, tightened up the sagging cable, ran a fresh and shorter cable through my wall and now the modem indicates a signal that probably couldn't be much better... no alerts in the log, much stronger signal strength, and almost no correctable errors at all.  Unfortunately, the issue remains the same as before.

I've tried replacing the cable between the modem and the router.

I've tried rolling back the firmware in the router to a version that it was running the last time this wasn't occurring.

It's not just one of the LAN ports since 2 simultaneously connected devices have the same issue at the same time.

And it can't be the WLAN interfaces since a wireless connected device has the same issue at the same time.

It's got to be either one of the ports interconnecting the router and the modem, or it's the switching function or the routing function running in the router itself.  The alternating 30-60-30-60 minutes interval has me baffled.