I already tried replacing the cable with a known good cable yesterday, as well as putting that cable through a tester. (I do networking as a career so I have a bunch of tools.)
The modem I have is somewhat flaky. Ever since I bought it (2 years ago?) the internal web server stops working after so long of uptime until you power cycle it. Other than that, it has always worked fine.
Anyways I've had dstat running nonstop, and I've only seen one outage since the last one I posted. The problem may have gone away, I'll check it again tomorrow.