One of my Minis was running hot continuously. After a lot of checking, I finally used Activity Monitor to see what apps were doing what. Dropbox was the culprit using about 113% of the CPU time. I checked the help pages to no avail and finally simply quit Dropbox and restarted it later. So far, so good.
Reboot - After all these years one would think I would have thought of that first. Oh well…