I had a similar problem with a different ISP, Optimum, in Northern NJ. It wasn't as regular as the author's problem -- my cable modem would desync intermittently throughout the day despite the signal strength numbers being in spec.

I replaced everything downstream of the drop from the street, all new wiring inside, a new modem/router/etc. All signs pointed to the problem being outside the house. I went so far as to connect an oscilloscope to the coax line to look for patterns. I discovered that if I physically manipulated a particular section of the line from the pole, a huge interference pattern appeared and the modem's connection dropped. Eventually I could reproduce the connection loss fairly easily.

Convincing the ISP to actually do anything about it was much harder. Despite first-hand evidence that the coax from the pole needed to be replaced, their tech support insisted that someone had to come into the house to inspect the interior wiring. No amount of insistence on my part would convince them that it was not necessary. The building was a vacation home, and this was during peak COVID time, so there was basically no chance of that happening. The appointment came with threats of service charges if they sent a tech and could not enter the building or reproduce the problem, so I cancelled it.

Coincidentally, I happened to discover that the mayor of the town had started a hotline specifically for reporting home Internet problems in the town. So I sent in a message to that service, not really expecting anything to come of it. But shortly after I get a phone call from some higher-up department of the ISP. They had a truck out within a few days to replace the drop -- with no one home -- and the connection was rock solid ever since.

This experience taught me that ISPs often have distinct support channels that governmental departments use to contact them. I think they called it the "executive support team" or something along those lines. Basically, if you can get a message in that way, it's possible to circumvent the useless consumer-level support. Long story short, I think escalating this through the local or state level government may be the author's best shot at getting this resolved.

ISPs also have different levels of service for different entities, and seem to just barely care about you as a customer.

An ISP (like one that starts with the first letter of the alphabet and ends with a common abbreviation for an explosive compound) might not think it’s worth coming out and marking their fiber lines when you call the city’s 811 number to mark utilities before digging for a project, like a fence.

If that fence ends up cutting the fiber line when digging a post, the company installing the fence can submit a ticket through a different portal than you as an actual residential customer of the ISP can, and that ticket probably gets responded to well before your attempts to contact them and request a call back because they are always experiencing a high volume of calls.

They’ll never admit any negligence on their part for refusing to mark utility lines, and you just have to remember where they buried the new ones, if they ever came back out to bury them instead of just leaving them aboive ground and flailing around.

Sometimes they even try to charge you for fixing the fiber line.

These days you get a lot better result from any company if you take a few minutes, find the email of a few VPs in the target company, and write the execs directly.

Exec fowards the email to the correct underling with "WTF?" added to it. You get phone calls the next day.

I’m wondering how you used an oscilloscope to diagnose the ~1ghz bandwidth DOCSIS signals on broadband cable. I have a (expensive!) gigahertz bandwidth scope but I’m not sure what I’d look for on it if I connected it to my cable.