I have a late 2012 Mac MIni, and I really am happy with it. I did, though, swap out the slow (5400 rpm), 1 TB internal hard drive for a much faster, 256 gig Samsung 840 Pro SSD, and it sure makes a difference!
In any event, first, how long has this issue been going on?
Secondly, you need to make a backup to an external device. The Mac OS comes with Time Machine, which is fine for backups. I myself, though, use SuperDuper! for my backups, as it creates a bootable clone. Similar software has been prevalent in the Windows world for quite some time, so maybe you are already familiar with such products.
Apple did release V10.12.2 of Sierra a couple of days ago, and you can get it form here:
https://support.apple.com/kb/dl1900?locale=en_US
Note that is the "combo" update. It is (most of the time) better to apply the Combo Update, versus an incremental one (Appel also released an incremental OS 10.12.2 update).
Not sure if installing it will resolve your issue, but it's worthy a try (AFTER you have the backup).