Found it. Can't believe it took this long. The C1 ram slot or signal path is damaged. It wont boot at all when overclocked with a stick in that slot alone, boots occasionally with 3 sticks in but rarely loads windows, and I get erratic boot errors when running stock speeds with that slot populated (eg: bootmgr is corrupt or missing etc).
Board will be exchanged for another. Hope the next one works!
Thanks for the advice guys, nice to have people to bounce ideas off

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