Easy one that I spent far too long on today. I'm not sure but I'm pretty sure this PC is one of those "bought on FB marketplace" jobs, and was advertised as just "Intel i7 3.8GHz" with no mention that it's actually a Q1'11 CPU.
Spoilers below...
It was a bad RAM stick. Other one was fine. Mobo was cheap and nasty - but it works.
_____________
Thanks to music bros down below!
Synthis - https://www.youtube.com/user/samsynthis
Ertrii - https://www.youtube.com/user/ertrii