I restarted a little while ago into memtest. DIMM#2 in Slot #1 showed a lot of errors. Over 500 before test 4 was complete and it looked like the number was still rising.

I've now removed that DIMM and reinstalled DIMM#1 into slot #1 as a sanity check. I will run this module longer than I did the first time and under greater load. Once I've done that then I'll run it in slot #2 to complete the whole test pass and help make sure I'm not actually seeing two failure points (mobo and memory).