So, as much as I'd like to ship off a few dozen of these babies we've been holding off due to lingering apprehension about power. I sent some cards to Barefeats.com for testing. He got FANTASTIC results, but was able to replicate my power issue.
Specifically, using any of the 3 cards (1080Ti, Pascal Titan-X and Pascal Titan-Xp) and running the CUDA-Z benchmark isn't a problem...until you hit the "Heavy Load Test" button. Then, with the Titan cards you will almost ALWAYS have an immediate shutdown. (within 2 seconds). With the 1080Ti this will sometimes require upwards of a minute, but still MOST will eventually shut down the machine. Oddly, the former "Power Virus" known as Furmark can be run for any length of time with few, if any, shutdowns.
Even more perplexing is that Hardware Monitor shows LOWER numbers than with the 980Ti and Titan-X Maxwell. So the power draw must be a very fast spike, faster than the 1-2 seconds refresh for the sensors.
I was hoping the issue was my aging 4,1/5,1 which has tested MANY cards. But Rob has found same behavior and posts here and at Netkas have had a few other people report the issue. In addition, one customer has reported that a 1080Ti he received will shut down the machine playing UT. He tested in other high draw situations and found the card stable.
We have found a solution that was tested and confirmed working by Barefeats. A cable that connects power from a SATA drive makes the computer continue running no matter what. I have found no way to cause the power issue using this cable. Some may not like this additional cabling. Sadly, the only other choice is to stick with a 1070 or 1080 (or two) or use external power.
I look forward to your thoughts and experiences. In fact, most people may never run into this issue using the Dual 6 pins inside the machine, but I need to make everyone aware of this before they buy. For the few we have already shipped (less than a dozen) we will offer either the additional power cable or a full refund (no 10% restock).