Problem with old MBP

[clv101]

I've got a 2012 i7 MBP here and it won't boot. It chimes, Apple logo show, progress bar gets half way then it reboots and just loops like that.

I can't boot into Safe Mode.
I can boot holding Option-S for single user mode and run fsck. Reports the boot volume 'appears to be OK'.
I can boot holding D to run the Apple Hardware Test and everything seems fine.
I can enter Startup Manager (holding Option) and select a USB stick with OSX install on, but attempts to boot from this result in the progress bar followed by a frozen white screen.
I can use Shift-Option-Command-R to start up from macOS Recovery over the Internet, after entering WiFi details, back to a frozen white screen.
Booting with Command-V for verbose mode doesn't suggest anything obvious...

Any ideas?

 

[clv101]

That was my first thought, but fsck checks out and the Apple Hardware Test doesn't find anything wrong.
In single user mode I can navigate the whole file system.

 

[clv101]

Swapped in a known good drive - exactly the same deal.
What else can I do to figure out what's wrong / fix it?