2013 iMac starts beachballing on its own after a few minutes

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?

I am no expert, but here are a few ideas:

  1. A memory leak slowly eating up available RAM? Keep Activity Monitor up front on Memory Tab tab and sort by memory.
  2. A thermal issue? Some internal device heating up and slowing or even stopping. Does it work for longer from a cold start after it has been powered down for say 30 minutes?
  3. A background process that starts 10 minutes after boot? Keep Activity Monitor up front on CPU tab and sort by %cpu
  4. Badly behaved app trying to sync.? I seem to remember a horror story about DropBox doing this.
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Dying drive.

Had similar on a 2015 MBA. New SDD fixed it.

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That was my first thought - have you ever swapped the drive out @AVC?

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Nope, I thought it probably was a hard drive issue, we’re taking it to an AASP tomorrow, I guess they’ll confirm that.

A friend had this issue with her 2013 27” iMac, I tried every trick in the book to make it behave but in the end we took it to Apple and it was a dying drive, indeed. Drive replaced and all is good.