macos – Prohibitory Sign Appears Sporadically

There is a popular question about what to do when the prohibitory symbol (stop sign) appears on a Mac. For me, none of the solutions work, but sometimes my iMac just manages to load. I’ve lived like this for years (I don’t reboot for months). I turn it on, reach the symbol, turn off, rinse and repeat. Within an hour it manages to load and works fine, and sometimes it loads immediately. There’s no pattern.

Notably, a clean install didn’t help with this.

Things that I’ve tried: resetting NVRAM, disconnecting power for a minute, verify disk and repair from the disk utility that can be loaded with cmd+r (there are no mistakes).

My vague understanding of the symbol is that it is failing to find where to load the system from – but why would it be able to load it occasionally?

Any thoughts on what could be the problem?

Thanks in advance.

Catalina v. 10.15.7 on a late 2013 iMac