You really got me thinking, JorgeB. Since the mobo is the only real "new" equipment in the equation, I went looking for the latest BIOS on ASUS' site.
It has that crypic, "Improve system stability" in the description. In my experience working in software development, that's often code for, "something was breaking regularly, we don't want to say what, but we did try to fix it."
I've updated the BIOS as a first step and I'll report back if that improves things or if there needs to be a next step. Fingers crossed.
Incidentally, having this support forum as a sounding board has been wonderful. I often mull these things over in silence, but collaborating with some other people with more experience has been great thus far.