Mac Pro 6,1 is completely dead after sitting off all night
I have one Mac Pro 6,1 Trash Can that is completely dead in the morning after being shut down and sitting all night. Everything is plugged in (2 HP Dream Color monitors, 1 Wacom Tablet, 1 Sonnet 10GB TB adapter, 2 USB3 external drives...one SSD, one rotational, both have their own power supplies)
I have to unplug everything and let it sit for awhile, sometimes it will power on after a few minutes with nothing attached, sometimes, it takes hours, but it always eventually powers on...and once powered up, it consistently works until I shut it down again.
As a professional broadcast facility, you can see the need to have the system work every time, not maybe today, maybe tomorrow...
I have tried running Apple Diagnostics, but the system sits at 1 or 2 minutes remaining...this is with either all 64GB of Ram installed or just 1 stick in there. It is quite possible that same stick is the stick that's holding the test up, but a second MP 6,1 I was also testing Diagnostics on hung at the same 1 minute remainig...this was left going over the weekend, so it had plenty of time to get through the test.
It has been sent in 3 times to the Apple store and they haven't found anything wrong (minus a false positive bad stick of Ram last time, which is why I'm running MemTest now). I have escalated the issue and they are awaiting me to send it back one more time. I am running Memtest at the moment to see if it picks up anything, and would love to get my hands on the software Apple uses to diagnose hardware issues, but they seem to only be available to Apple Certified technicians under penalty of death...
Any other ideas before I go through the agony of sending it back to the Apple Store?
Mac Pro (Late 2013), OS X El Capitan (10.11.6), 64GB Ram/D700 GPUs