What is NVRAM on the Mac computer?

Non-Volatile Random Access Memory, essentially also called Firmware it holds setting needed to boot your computer, like where your boot drive is, what kind of Monitor, sound settings, & security settings, etc., also called PRAM, Parameter Random Access Memory.

It holds those settings when computer is off, usually via PRAM Battery.

Suggest running EtreCheck and posting the output so that someone can analyze it.

https://etrecheck.com/

Paste the EtreCheck output in an Additional Text box

Sounds more like a failing hard drive.