Postmortem update – Problem resolved:
I wish to thank those that contributed their insight into my system issue and report what I’ve determined what the underlying issue is.
Apparently, the Dell XPS 9000 power supply is sensitive to UPS power backups that do not provide pure sine wave power. I had my system connected to Tripp Lite standby backup that apparently provides marginal power for this computer. I had misled myself into thinking that the issue was NOT power related because I could remove the power from the wall and the system seemed to run fine on standby power, and secondly, the AMI BIOS option to NOT restart after power loss is enabled.
As it turns out, the system disregards that BIOS option and reboots if power is lost for a brief interval, something like one second. Additionally, the Tripp Lite seems to occasionally go into some periodic maintenance mode that results in the Dell PSU sensing momentary loss of power.
One day I happened to hear the click of the Tripp Lite doing its thing and at the same time witnessing the system restart. Further investigation confirmed the faulty behavior of the BIOS option. Of course when I spoke to someone in the business of repairing UPS backups, they confirmed the issue with the XPS PSUs and also said that high-end Macs have the same problem. After removing the system from the Tripp Lite, all is well. An industrial grade APC backup with pure sine way output is in the mail.
So there you go…
Jim