So unfortunately I spoke to soon! After completing the build with the replacement board, everything appeared to work fine. I installed Windows 10, got the chipset drivers working, had several normal reboots and I'm thinking I'm in the clear. Then, after maybe an hour the exact same behavior began again... the board shuts down, the CPU error indicator turns on and the CPU fan cycles on and off for 30 seconds or so, then the machine boots right back up as if nothing is wrong (on the last "defective" board, the machine would NOT reboot).
This cycle repeats randomly. It might take an hour, it might take 15 minutes for the same behavior. The CPU temps are fine. I ran a load test and under 100% load the temp maxed out at 60C. I cannot figure out what this could be other than a bad motherboard, but given this is my 2nd build with the same board and now same symptoms, could this be something else? Would a "bad" CPU cause this or could there be a PSU issue?
My only idea to troubleshoot is to try the same build with a different motherboard to rule out something else, but I'm at my wits end here! any advice would be most welcome.
UPDATE: I ran an Intel CPU Diagnostic Utility and everything was fine with the chip...