Everything came ready-seated apart from the cooler, I had a lot of long-story fun and games with that too. I trust the seller and that it was working for him.

Tried a few different BIOS settings today, nothing major. Nothing changed anything, so I put 'em back. Then I remembered this switch on the board which enables the TPU - I'd turned that off during earlier woes but the original owner had it enabled originally. Anyway, since it effects overclocking I figured why not try and enable it, the temps are all nice and low it might even solve the RAM thing. 

It wouldn't even get to BIOS. Erk! I set it to the middle setting, and again no good. I turned it back off, and I still can't get the fucker to boot. Fucking hell, computers. Resetting the CMOS via the switch didn't help either, it would get part way through the boot and then seem to hang, but not always in the same place or with the same Q-code. So I pulled the battery for half an hour and then fired it back up.

It got to BIOS. More weirdly still, it shows 32GB RAM!

I enabled the XMP setting again since the reset had turned that off. It still shows 32GB RAM! And in Windows!

I think that probably means DSMITH is the prize winner, it's just a shame I solved it myself before I read that :)

I did try all that reseating the CPU stuff yesterday having read the same forum posts, it didn't help me.

Is it still stable, after reboots and a total power off?
Might be worth looking around for an updated firmware for the board. There was one site that I found really useful when sticking a custom firmware onto my ageing board which I needed to get a recent GPU upgrade working. I'll post the link if I can find the site again.

seems fine now. Firmware was all up to date. I guess it just really really needed the CMOS battery reset for some obscure computers reason. Next thing to try is overclocking a lot!