I am having the same problem guys, and it started … I don’t know why. I installed still the High Sierra, but noticed that was too slow and did not bring any new features. I went back to El Captain, which runs “perfectly”. but since about 2 months, the macmin keeps ramdonly restarting. In hardware did not touch since abot 3-4 years is running with the same one and as no changes, I can not believe something is wrong with it! Hardware is simple, works or not working. I did a 2nd partition on the SSD and re-install new El Captain, without any external software, and it keeps doing it … now my next step is erase completely the SSD, as hard as possible, and try to install the original Lion … and go from there, that I will do it this week, meanwhile I am assuring no loss of data before the erase operation