This started happening a couple weeks ago when it began shutting down whenever it went into deep sleep. Disabling deep sleep kept the problem at bay for awhile, but then it started shutting down while using the computer and performing disk intensive tasks (I was installing Adobe CC suite). Resetting SMC and PRAM again seemed to keep the problem at bay... for a few days. It doesn't matter what OS or which internal drive I boot off (SSD or one of the regular 3.5" drives): it will shutdown whenever it goes to deep sleep or typically when performing disk intensive tasks such as large file transfers or big software installs (such as a clean OS X install). It may also shutdown when using external USB drives or other high-power draw devices (e.g. Corsair K70 Lux with its two USB plugs). I pulled all hard drives and booted my primary startup one off a USB drive dock, and although it runs for awhile, it will eventually shutdown; it just doesnt happen as frequently. A quick AHT showed no issues, but when I started the long test, it would always crash about 10 minutes into it. CPU temps are typically sub-60 degrees Celsius, and the heat sinks aren't painful to the touch heat-wise. I removed all RAM chips (have 8x8GB) and swapped in different pairs but the problem persists. I've got an Apple Genius Bar appointment for the weekend but would like to solicit ideas as to the culprit: I think it's either Power Supply, one or both 3.46 CPUs, or motherboard. Suggestions?