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Very strange issue here. My Mid '11 Mac Mini (on OS X El Cap, 10.11.6) is randomly rebooting during the day and (mostly) at night when not in use. I will be up in the middle of the night getting some water and hear the Apple chime. Worth noting, I have never seen it reboot while I am using it.

I've searched for anything that says "kernel" in the Console program in OS X, but all I see are "PM kernel: Sandbox" items...are those related? Any help here much appreciated. Thank you!

[EDIT] After running the command syslog | grep -i "shutdown cause"from Terminal, I get 271 instances of the unexpected reboot over the past 5 days, all with cause of -128 

Oct 28 11:02:59 localhost kernel[0] <Notice>: Previous shutdown cause: -128
Oct 28 11:37:54 localhost kernel[0] <Notice>: Previous shutdown cause: -128
Oct 28 12:07:55 localhost kernel[0] <Notice>: Previous shutdown cause: -128
Oct 28 12:35:52 localhost kernel[0] <Notice>: Previous shutdown cause: -128
Oct 28 13:56:05 localhost kernel[0] <Notice>: Previous shutdown cause: -128
Oct 28 15:52:22 localhost kernel[0] <Notice>: Previous shutdown cause: -128
Oct 28 16:03:45 localhost kernel[0] <Notice>: Previous shutdown cause: -128
Oct 28 16:26:52 localhost kernel[0] <Notice>: Previous shutdown cause: -128

This repeats many, many times with entries throughout each day. Click here to see the full log

Allan
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SamAndrew81
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    Look for a message that contains Previous shutdown cause, for example: Aug 8 22:00:23 localhost kernel[0]: Previous shutdown cause: 5 – MacManager Oct 28 '16 at 19:56
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    You need to check for the shutdown cause - issue the command: syslog | grep -i "shutdown cause" Then post the output to your original question. – Allan Oct 28 '16 at 19:57
  • Thank you both. I ran the command in Terminal and it returned MANY of these (about 15 or so) for the past few days, all with cause listed as: -128

    Oct 28 00:20:03 localhost kernel[0] <Notice>: Previous shutdown cause: -128

     – SamAndrew81 Oct 28 '16 at 20:40
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    According to this this link, the Mac is overheating or it might be 3rd party RAM (which I do have installed): https://discussions.apple.com/message/18098171#18098171 – SamAndrew81 Oct 28 '16 at 20:43
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    I have actually answered this very question before - see this answer: http://apple.stackexchange.com/a/244285/119271 – Allan Oct 28 '16 at 22:43
  • Thanks, @Allan. I ran the AHT twice and the test results were "Not trouble found." Pic here: https://goo.gl/photos/jMd7V5VTJTTHkSFD9 – SamAndrew81 Oct 29 '16 at 17:47
  • *No trouble found, typo. – SamAndrew81 Oct 29 '16 at 17:58
  • Maybe it is just overheating? I have it sitting on top of my old Mac Pro 1.1, but I usually have the MP off. I'm trying setting the Mac Mini on its side to see if that helps. – SamAndrew81 Oct 29 '16 at 17:59
  • BUMP. Can anyone help here?? – SamAndrew81 Oct 31 '16 at 23:36
  • Still stuck here. Probably has actually worsened and the Mac Mini boot loops at the Apple Logo now. However AHT says no problems found and I can boot to Safe Mode fine. – SamAndrew81 Nov 07 '16 at 16:04
  • Thank you, @Allan. Wipe/reload of the OS fixed the issue, but your feedback definitely helped me. – SamAndrew81 Nov 09 '16 at 04:43

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This ended up being a software issue. A clean install of the OS fixed the issue. I could not get the recovery partition to boot, so I had to use another Mac to create a bootable USB installer for macOS (downloaded from the App Store).

Worth noting, I was able to put all my apps and settings back into place via Migration Assistant, BUT only after booting into macOS (not on first boot during setup where it prompts for Migration Assistant - it failed there for some reason).

SamAndrew81
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  • NOPE, I was wrong! It was sleep that was causing the reboots!! I have not just set the Mac Mini and the monitors to NEVER sleep (I turn the monitors off and/or use a screensaver) and the boot loops have stopped. Worth noting that a clean install of OS X did seem to get the initial boot loops to stop (when powering on the Mac Mini), but I think a SMC reset and/or PRAM reset might do the same, not sure. – SamAndrew81 Nov 10 '16 at 17:17