This is just a follow up. I have now replaced the motherboard with a SuperMicro X10SL7-F (along with the CPU, and memory). System is back up and running fine. The old CPU and partial memory from the previous unRaid setup is now being used in another media machine and it too is running all fine.
Solution: bad motherboard - note to others that may be having similar crashing issues. I was fortunate enough to have enough spare parts gathered over the years to be able to troubleshoot hardware in other setups to determine if any of them were the culprit.
