PC randomly turns off as if someone pulled the plug

PC can be running fine then just totally switch off. Last night it came back on itself after maybe 30 secs, other times it didn't.

It's a brand new 13900k from Intel as I thought that was at fault, it was replaced. So it's not CPU.

Tried different memory too so don't believe it's that.

All worked fine with another MB and CPU, so ruling out drives, PSU and GPU.

Seems likely the MB is at fault, but is this a symptom a faulty MB can cause? Or am I missing something?

MB is an Asus Z790.

EDIT: Forgot to say that it resets the BIos each time too.