Modem remains connected to Cox, but no data can get through; reset solves the problem

I live in 85020 and have the Internet Ultimate Classic 300 package from Cox.  I do not appear to be anywhere close to the data caps for this plan.  I have a Netgear C7800 Modem/Router combo, purchased in April 2020.  It is running the latest firmware.  When it is functioning, the C7800 delivers great connection speeds, often over 200 Mbps down, 30 Mbps up.

However, for several months now the connection to Cox will periodically close off, and essentially stop allowing data to pass through.  The modem shows that all 32 of the downstream channels are "locked" with very few errors, but none of the devices attached to the network are able to get data from the internet.  Local traffic on my home's side of the connection (both wired and wireless) is unaffected, so I do not suspect the router.   This problem will sometimes manifest itself as an inability to make a new connection.  For example, a child with an active Google Classroom video chat at the time the problem occurs will continue to be able to send and receive data, but another child who is trying at the same time to click on a new lesson cannot connect to the internet.  Or a TV will stream part of an episode of a show, but will be unable to reconnect to get the data for the second half of the show.  I refer to this as the "slow crash" - it affects some people on the network but not others.  This creates plenty of arguments in the home when one person can't connect and needs to do something urgently so they ask to reset the connection, while other people remain connected and don't want to have their connections reset.

Eventually any lingering connections will be choked off as well, and no data will get through (but I have seen the slow crash take as long as an hour).  Rebooting the modem solves the problem, but it has been recurring consistently over the past three+ months.  Sometimes the modem will pass data without needing to be rebooted for as long as 36 hours.  Sometimes the crash occurs again within minutes of rebooting it.  Also, please note that I follow the manufacturer's guidelines by either 1) rebooting the modem using the Netgear app, or 2) unplugging the modem and letting it sit for at least 20 seconds before plugging it back in.  The problem recurs after either method.  The problem does not appear to be related to the amount of traffic on the modem at the time, or the number of devices that are actively using the connection.  I have had the problem occur overnight when no one is using the network, requiring me to reset it when I wake up.   I have had the problem occur when I was the only one home, and find myself unable to get a simple iMessage to go through.  Conversely we have had days were two adults and two kids are all streaming different videos, online gaming, zoom calling, etc. at the same time with no issues.

I have "reset" the modem using Cox's support option on at least two occasions, but the problem recurred each time.  (To be honest, I don't use Cox's reset often because it takes up to five minutes to come back into service, while my modem's reboot takes 90 seconds.)   I have also had a Cox tech out to my house and he determined that there was no issue with the signal strength on the wires to the junction box in front of my house.  He refused to come inside and check the strength of the signal at the end of my coax cable (Covid protocol, he said).  This is a rental home, and the coax was here when I moved in.  The cables look good to me, but I don't know their age.  The Cox tech said he would ask their engineers to look at the data stream to see if they can identify the problem, but I never heard back from them.

I posted screenshots of my modem's connection information.  The first is what the connection looks like when it is "locked", and the second is what it looks like when things are functioning normally.

I have Googled this problem several times using different terms and can't find a documented issue with C7800s or Cox connections behaving badly like this.  I would appreciate any help anyone could provide with diagnosing this problem.