Blah. After running seemingly without issue for quite a few days, the machine restarted tonight and showed a lot of errors on the module I thought was likely ok.

Turned the system off briefly (obviously not long enough) and it ran for about 2 hours before crapping out again.

I've since shut it down for about 5 minutes and verified that a new run of memtest isn't producing errors.

I think I'm going to RMA the memory and the mobo now.