Silly question but must ask, you did reinstall Windows after the board swap?
EDIT: If refering to bootable memtest please ignore me
No I haven't reinstalled Windows yet, but it was indeed bootable memtest86+ v4 from CD so that rules that out. Besides, the Windows memory diagnostic is a boot-test anyway so surely that wouldn't affect the results?
I guess I can only RMA the OCZ sticks and see what happens. I'm quietly going insane in the meantime.

