Well, the unit passed all memory tests, all was well, zero errors across the board. Shut it down, swapped the memory for the OEM stuff, rebooted.
System init took longer and fan behavior was weird - rear fan would occasionally spin then stop, etc. before coming on 80% as per BIOS setting. First time, the unit made it to "syncer" on the console screen before it rebooted itself. Some more back and forth, now the unit claims to be running but the web server for the FreeNAS is down (yet the IPMI server is working just fine). Voltages look OK.
The CPU temperature is below 40*C so that's not it either.
So I guess my next step will be to nuke the LAN interfaces again and see if that fixes the problem.