Mac Mini Server Shuts down every 14 days for no apparent reason.

Question

Just before 8 AM every other saturday morning, I have a Mac Mini Server running 10.8.5 server that powers off. There is no indication of anything out of the ordinary happening prior to the shutdown. On restart the server continues to operate properly for another two weeks until it shuts down again. The only log entry I'm seeing that seems to indicate what might be going on is:
Dec  7 22:51:29 localhost kernel[0]: Previous Shutdown Cause: 3
This past weekend I attempted to get around the issue by having the system set to automatically turn on or wake at 11 AM, but that did not happen.
I've checked cron and launchd and nothing is scheduled around that time.
Any ideas?

Answer 1

Yes, a glitch with any less-interruptable power supply — they're never truely uninterruptable — could well trigger this outage. 
If you're willing to run with grid power for two weeks plus a day, then cable the APC out of the path, and see if your uptime makes it to fifteen.   
Probably much quicker and if you've got some off time where you can quiesce most of the activity on the Mac to allow it to crash hard with less risk to your data and your processing, then pull the input to the APC and see if the Mac crashes.
Various APC units run a self-test every two weeks, so a problem with the self-test or the switch-over or with he battery could well cause the reported problems.  Could also be the quality of the power being generated (I'm running Smart-UPS units because of that) or the APC is overloaded, but I wouldn't expect that to occur on a particular schedule. 
This reeks of the self-test or the power relay, or the battery.
Some semi-related discussions here, and probably elsewhere on the net.