My venerable MacPro3,1 running El Capitan often suddenly shuts off to a dark gray screen, and then immediately restarts. Often, this is a double-restart, with the second following the first immediately, with nothing appearing on the display between the two. This has only occurred at, or shortly after, first booting the cold computer. Once it has run for a while, it has not (at least to date!) had this problem after running long enough to warm up. I have tried resetting the power manager and NVRAM. I even bought a new battery backup, since I knew the batteries in the old one were dead, and suspected summertime micro power failures as the culprit, yet the problem remains.

Apple Hardware Test found nothing. This computer occasionally dual boots into Windows 10, off a separate SSD (sorry, fans; it was that or buy another computer for software needed for classes I am taking). I am currently booted into Windows 10, to see whether the problem recurs, as it would be clearly diagnostic if it occurred only under MacOS from the MacOS SSD. More news on that after Windows has had time to run, or, more likely, the computer has had time to cool completely, and then start directly into Windows.

Years ago, I had a Beige G3 with a similar issues that was fixed by replacing bad RAM. Any chance that is the issue?

Any idea what might cause, and how to fix, the mysterious restarts?