XPS 8930, 2-2 code

Very strange failure.  Began when I returned to the computer after a brief pause in work to find it repeatedly clicking, attempting to restart?  After several tries, I got a message that the system needed to be repaired.  I followed guidance, ran system repair.  No specific problem found, computer rebooted and operated normally.  Several days later, similar problem: this time there was a message that my boot drive was overheating (it wasn't), it rebooted normally, and I continued to work on it.  I took some advice and replaced the M.2 SSD in case it was failing.  Several days later, similar problem after working 8+ hours on it, left the room and returned to find the clicking/restart behavior.  This time it didn't restart at all, gave me the amber 2-2 code.  I removed/reseated the RAM, then replaced the RAM with new RAM, then replaced the mother board, then replaced the power supply, and finally replaced the Intel I7 processor.  I've basically rebuilt the computer, and still get the 2-2 amber code when I plug it in.  Any idea where to start?  Can anyone recommend a good troubleshooting guide where I can begin with the basics, figure out what works and build it up to a fully functioning computer again?  I'd give up - but honestly have so much time and money tied up in the process that I don't want to quit on it!