XPS 8900, Intermittent POST/Boot failures, possible solutions

I want to share a solution that worked for me. Maybe it will work for someone else too.

Problem:

For about two years, I've been fighting problems where my XPS 8900 would INTERMITTENTLY fail to start.  It could be a Restart, or cold start (Powering On using the Power Button on the chassis).

Either the monitor would continuously display the round Dell Logo, or the display remain black/blank.

While the color of the Power Button was white, indicating POST (Power-On Self Test) was successfully completed, there was no indication of Hard Drive activity (no hard drive noise or white activity light).

Repeated diagnostics, OS reinstalls, and BIOS updates made no difference.  Running with RAID-1 or without RAID made no difference.

My Operating Conditions:

• Running Legacy Boot rather than UEFI ( per Dell tech support, Legacy required to setup and reliably run mirrored Hard Drives--RAID-1).
• Using Seagate 2TB USB external Hard Drive for data backup and for storage of Windows System Images.  USB cable continuously connected.  Power controlled by Uninterruptable Power Supply (UPS)--When PC is drawing power, external drive is energized, when PC is shutdown, UPS disconnects power to external drive.  
• Living in a rural area where power often has very short interruptions ("blips" that cause lights to flick off momentarily), I used an APC BR1000G (stepped AC waveform) UPS to condition power.  This UPS, and others like it in prior years, worked great when running my old XPS 8700 (Windows XP) PC.

Solutions:

• Thinking that some sort of "race condition" was going during the POST/Boot phase of PC power up, I removed USB devices as a Boot Option in the BIOS.  This change in BIOS alone may have solved the problem.
• I don't know if having stored Windows System Images on this USB drive contributed to the problem or not.
• Thinking that the UPS may or may not be contributing to the problem:
  • First, reconfigured APC BR1000G UPS and Power Connections so that no devices were automatically de-energized when the PC shut down. All equipment connected to the UPS, including the USB Hard Drive, now had continuous power applied.
  • Second, having heard that some PCs are more sensitive to AC power conditions, I replaced UPS BR1000G (stepped AC waveform) with model BR1000MS (true AC sinewave).

Recommendations for Troubleshooting Intermittent POST/Boot Failures:

1. If there are USB Hard Drives (or thumb-drives for that matter) that are normally left plugged in, uncheck "USB" found in the BIOS Boot Options list.  Make sure the primary hard drive is checked and listed first in the startup sequence. I left the CD/DVD/BluRay drive checked, but listed it second in the startup sequence.
2. If there is any question about the quality of power going to the PC, use a UPS that has plenty of reserve power for the PC, Monitor, and other devices that may be powered through it.  While 10-mins of reserve power provided the UPS may be adequate, I prefer a minimum of 20-mins to compensate for the standby battery pack in the UPS having reduced capacity as the cells age.
3. If you are using a UPS that has a square wave or stepped sinewave AC output, consider replacing it with one that has a "true" or "clean" sinewave AC output.  The prices and availability of UPSs with true sinewave AC outputs have improved much in recent years.

Don't know if this helps anyone or not, but with all the trouble I've gone through over the last two years, I felt the fix that worked for me should be shared.