Sometimes there is more than one problem cause, or several things compounding the diagnosis.

Coax cable can go bad. Just from age, mechanical damage from being crushed, abraded, kinked, flexed, etc. Or from moisture getting into the connectors or under the cable sheath.

Often a degraded coax will allow the cable modem to work sometimes (even most of the time), but the modem will internally struggle to maintain the connection. Packet loss rates may be high yet the Internet access seems to be mostly 'ok'. Yet the modem will randomly lose the link, and may or may not be able to quickly regain it.

I just went through such an episode where multiple on site cable techs measured and tweaked signal levels, checked noise levels, and of course replaced the cable modem. Eventually (after several months) a short segment of old coax that linked the inside coax wiring to the outside demarc box was noticed and replaced. When it was removed, the old coax was found to have been pinched behind a metal pipe. The sheath had been abraded, allowing moisture inside...

In my case rebooting the equipment (modem and/or the separate router) would sometimes restore service right away, sometimes not.