keep mac mini mid 2010 el capitan from shutting down

I have tried different surge protectors and outlets. This machine has been in two completely different environments, with different sets of software from different sources.This machine has been rebuilt several times.Again, they can't get the machine to replicate the problem. I brought back to Apple once more and eventually spoke on the phone with a master tech somewhere in the Midwest who also thought the 'Geniuses' must be missing something and gave them more stressful test to try. At first just every few days, then nearly daily (oddly often around the same time in the afternoon), and then multiple times a day. This time it worked for a few months before starting to shut down occasionally again. We rebuilt the machine once more and tried again. They kept the machine this time and ran diagnostic over days but could not get it to repeat shut down in their possession and insisted it was a software issue. Within a week the Mac started shutting off again.

I reformatted the drive, did a new install and step up with new software in a completely new environment - I did NOT restore from any user or backup. Rather than try to deal with it over long distance, I had him buy a new iMac and took the old one with me to NYC. This problem persisted for months and grew more frequent until it was happening several times a day. It will simply power off completely at random - sometimes being able to be powered back on immediately, sometimes not turning on for an hour. This iMac was my father's and began having this issue last summer.

Keep mac mini mid 2010 el capitan from shutting down