Sorry, looks like it was always broke to begin with.
It has even been caught re-booting while the bios memory test is still happening
Then it's defo hardware, usually MB but it could also be mem or cpu. I remember having a deeper look at someones computer that was randomly doing strange things at all stages of operation and it turned out to be some of the MB PCB wiring had detached from one side - threw the MB in the bin and brought a new one for her, problem solved.