Okay, "problem" solved!
Having not really read (in the terrible documentation for my mobo) how to install the memory in the right slots, I had two sticks in the wrong slots.
When I popped them out and moved them into alternating slots, they worked just fine. (I'd read about this as a possible fix in another forum).
So, the problem was human error, which I would never have discovered had Snipergod87 not asked about Hardware Reserved Memory...
Thanks everyone!