Here are the basic specs: Mid-2012 Mac Pro 6-core 3.46GHz 32GB RAM Nvidia GTX680 4GB Sierra, High Sierra or Mojave 3 PCIe cards aside from the Nvidia—all have been removed to no effect. The problem began while stacking 30 photos in Photoshop, something I've done previously without any issue. The Mac simply shut off. I have tested it and it will do it every time. I tried performing the same function in safe mode and Photoshop simply froze up. Since then, my Mac Pro has been crashing on me continuously the past few weeks. It now shuts down unexpectedly within a few seconds to a few minutes or so after booting, and sometimes during boot! It doesn't happen in safe boot mode. I haven't been able to isolate what's causing it. It's been working perfectly for the past two years until now. I've run TechTool Pro a few times and it also found no hardware issues, all passed. Useless. I reset the SMC quite a few times after crashing which didn't seem to help, and I reset the PRAM as many times, also to no avail. Here's the most significant aspect as far as I can see: it does not shut down in safe mode when booting from any drive with any OS. I've been running for the past 36 hours or so without a crash in safe mode with High Sierra. I've reloaded some of the start-up items (which by the way I disabled for normal booting, and I have disabled extensions as well—no difference when booting normally. However, one thing that keeps happening in safe boot mode is the screen sometimes flashes when performing certain tasks, like writing in Word. Not sure if it's just because the Nvidia drivers aren't installed or what. I tried to install Mojave when in safe boot mode and it told me there wasn't a video card that would work. Booting normally allowed me to install it, although the Mac crashed 4-5 times during installation! I updated the Nvidia to the latest web drivers, still not difference. Everything works well in safe boot mode and that leads me to think it's not a hardware issue, unless it's the Nvidia display card. I am running a fan-control utility and all the temps are normal or below, nothing over 114°F . I have checked the heatsink without removing it and it seems to be solid and doing it's job as is the Northbridge. Now, I created a new volume on a fresh SSD and installed Mojave (the computer shut down numerous times during the install), and I didn't migrate any settings or apps, and no Adobe apps. I thought that fixed it but it was up for about 10 minutes and then crashed again. If it was the logic board I would think that it would crash in safe mode, same for the power supply. I've tried bypassing a UPS and plugging it directly in the wall, no difference. I've also changed the internal CR2032 battery with a brand new one. I have tried a brand-new Mac keyboard and a new mouse (back in the day I had a Performa that kept crashing due to a bad Mac keyboard). Nothing fixes it! I will install a new Radeon display card in a few days and see if that makes a difference but at this point I'm pretty discouraged. Any insight would be appreciated. Thanks.