Well, my machine did it again this afternoon, but this time it managed a reboot instead of just a hang. I returned my CPU to stock clock, and began running tests.
Both SSDs checked fine from chkdsk; I know this probably doesn't mean much, but they did.
Windows Memory Diagnostic ran, and hung at 99%. I heard the CPU fan back off when it should have completed the test, but the screen just sat there, accepting no input.
Ran MemTest86, all passes completed with zero errors.
Ran a quick FurMark benchmark, got the cards hot, no artifacts or hang.
I will keep the clock at stock, and report here again if I crash.