Thanks for your continued support here.
The shop is Scan Computers in Bolton. They told me they've tested the mobo, CPU and RAM and the entire thing has rebooted 25 times without error.
I'm fairly sure that in one of my many reinstall attempts, I've started the machine with only the boot drive (an SSD) connected. I'm in the habit of disconnecting the drives with all of my important files on them during formats, just in case. I see that the original poster in this thread was also experiencing problems booting from an SSD...