For closure, given I've taken up your time with this:
I think I solved the problem, though it's rather embarrassing. The first mobo, that got RMA'd, had this track damage, down by the gfx PCI slot. Well I took out the new board today and found almost identical (but note not identical - its not the same board that got mixed up somehow) scratch.
Can't be sure, but logical explanation is that somehow, after decades of doing this, I've suddenly developed some very sloppy technique for putting in the gfx card that was causing damage. If it is these scratches then its still a bit mysterious, because it took couple of weeks first time for problem to emerge, but second time was immediate. Also that i managed to twice do hardware damage that produced identical software errors is pretty impressive, in a Full Retard kind of way. And that I did all this with no sense at the time that I'd screwed up - not like I dropped a screw driver on the board and knew the impact had happened.
But its fits the facts I have so... Have ordered a new Asus board now, so if it does work will never know for absolutely certain it was the scratches, but will assume so. And rethink how I'm putting in gfx cards!