The Smallest Minority on earth is the individual. Those who deny individual rights cannot claim to be defenders of minorities. - Ayn Rand
I ran into a similar situation with Roadrunner. Connectivity got more and more marginal over time.
First, I replaced the Time Warner supplied splitter with a much nicer one. That helped, for awhile.
Then I replaced the splitter with a bidirectional powered splitter. That also helped, for awhile.
Then I had Time Warner come out and replace the line from the pole to the house, along with much better grounding (I had 60 Hz 0.5 V p-p on the coax shield!). That also helped, for awhile.
Then I replaced all the old coax in the house (RG58 and RG59) with new quad-shielded RG6, and new connectors. That helped for quite a while.
Eventually, the only way I could get that old cable modem to function was to hook it up directly to the cable as it came in the house, which was unacceptable, as it precluded using the incoming cable for anything else, such as TV.
I called Time Warner, told them I suspected I had a bad cable modem, and asked what was involved in swapping it for a different one. "Oh, just bring it in, and we'll give you a different one."
Sure as hell, that solved everything. Wish I had tried that first. It would have been quicker and cheaper.
The last time I went through this exercise, it took over two months before I simply could not get internet access - although the TV still functioned. It turned out that the cable from the trunkline to the house (which ran under the street) needed to be replaced. That was a couple of years ago.
This latest incident was the exact duplicate of the symptoms, but much more rapid in degradation. Plus, the service tech that came out on Easter Sunday had a TDR meter that showed the cable itself to be fine.
Which left the modem, pretty much.
Still working. No outages all day.
Yup, a couple of months back our up/down cable modem went down pretty hard, cable company came out and measured the line okay, forced a reset of the modem (some command down the line) and then it DIED two days later.
New modem, no problems since.
It turned out that the cable from the trunkline to the house (which ran under the street) needed to be replaced.
Hey, there's a reason I spent all that time, sweat, and money trying to make sure the cable modem had a good signal path. Das blinkinlights were still blinkin, and as far as I knew the FM(*) smoke had never been let out of the box. Besides, it worked most of the time.
(*) FM = "fucking magic"