Here is the latest:

I swapped out out existing risers with new ones. I had an unopened 6-pack of risers that I was saving for my next rig.

I then Only had (1) 1080ti plugged in and its works *perfectly* on its own without issue. In this state it was GPU #0.

I then plugged in the second 1080ti *also* with a new riser and only mined with this one. I set the configuration to only mine with the second GPU which came up as GPU #0 it too worked well on it's own.

I then kicked off in a separate window another instance of the EWBF Miner and specified only to use GPU #1 so GPU #0 and GPU#1 were both running in separate mining instances.

At that point VROOOOOOM!!! - the fans on GPU#1 spun up at 100% and the computer would become unresponsive but the fan on the GPU #1 still keep spinning at 100% until I forced a shutdown of the box.

The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.

So I am still perplexed but am also leaning towards a PSU issue.

I did look through the Event Viewer and did see this:

     Several of these:  "The system has rebooted without cleanly shutting down first. This error could be caused if the system
     stopped responding, crashed, or lost power unexpectedly."


     "The previous system shutdown at 8:16:18 PM on ‎12/‎7/‎2017 was unexpected."

I then forced both GPUs to mine and here are the Event Viewer entries:

     "The computer has rebooted from a bugcheck.  The bugcheck was: 0x00000124 (0x0000000000000000,
     0xffff8001358458f8, 0x0000000000000000, 0x0000000000000000). A dump was saved in: C:\Windows\Minidump\120717-
     8812-01.dmp. Report Id: 9d4fd139-4e1a-452b-ab24-d8f725a52999."

AND

     "A fatal hardware error has occurred.

     Component: AMD Northbridge
     Error Source: Machine Check Exception
     Error Type: HyperTransport Watchdog Timeout Error
     Processor APIC ID: 0

     The details view of this entry contains further information."

Any ideas?