This is an odd problem and not one I have ever seen before, but my parents’ 2013 27" iMac has developed a strange fault where it will start to beachball after about 10 minutes from starting up. This happens whether the computer is in use or not.
The computer just becomes totally non-responsive, and all you can do is restart and hope to get another 10 minutes or so until it starts again.
I have erased and reinstalled OS X (Catalina), and that seemed to fix the problem for a few weeks, but as of a few days ago, the same problem has started.
Does anyone have any ideas as to what could be wrong?