Okay, I rolled back to an earlier BIOS and it didn't fix the issue. After further testing I'm convinced it has something to do with my fans.
If I set fan Xpert to "standard" and boot up, just before the Q-Code switches from A0 to 00 as Windows starts up, my CPU fan revs up ever so slightly and then slows down again.
If I set Fan Xpert to "full speed" and boot with fans at full speed I get A0, until I set it back to "standard" and it switches immediately to 00.
I've now disabled CPU fan "extreme quiet mode" and was able to boot to A0 with fans at "standard," and I can switch between fan profiles without issue.
It's still early days but I think I'm on to something. I'll test more tomorrow. For reference, my CPU cooler is a Dark Rock Pro 2.
Greg W.
EASA CPL ME/IR