HI all
I got this resolved ... I'll post the results here, as I can see plenty of people have the same problem.
The quick story is that a BIOS update fixed it, using the BIOS flashback.
The longer story is ...
I got my mobo at the end of December 2020. You think that it would have a relatively recent BIOS (3001 was out on 08/12/20). After much googling and lots of isolating components, etc., I found a forum that said there is a sticker on the mobo with what looks like serial numbers. The last 4 digits are the BIOS version that was loaded when it left the factory.
My BIOS was 2404, released in July 2020, before Ryzen 5000. It looks like that mobo sat on a shelf for a while. A 5900x needs 2606 and above (released 17/08/20). So, I already had the beta 3801 on a USB key. I was planning on updating anyway, but wanted to do it from within the BIOS. I ended up sticking it in the BIOS Flashback USB socket and pushed the button. It flashed green for about five minutes, then went out. Lo and behold - it posted!!
Then I stuck all the rest of my RAM in, and eventually all the other bits I pulled out when trouble shooting. All good, and up and running now - yay!
That tip of the sticker showing the BIOS the factory installed was a life saver. I hope this helps others ...
Cheers
Doogz