Hey slog,
Have you checked the available space on your iMac to ensure it isn't almost full. If not, check out the steps here on how to check: See used and available storage space on your Mac.
If available storage is low, this article has information on how to free up space: Free up storage space on your Mac.
If there is ample amount of storage, make sure the macOS is up-to-date. Updates may include bug fixes. Follow steps below to check and update. Also, the steps are available here: Update macOS on Mac.
1. Before installing new software, it's a good idea to back up your Mac. If you have an external storage device, you can use it to back up your Mac with Time Machine.
2. To check for new software, open Software Update:
* From the Apple menu in the corner of your screen, choose System Settings. Click General in the sidebar, then click Software Update on the right.
* Or in earlier versions of macOS, choose Apple menu > System Preferences, then click Software Update.
3. If Software Update finds new software, click the Update or Upgrade button to download and install it. You will be asked to enter your administrator password, which is the password you use to log in to your Mac. During installation, your Mac might show a progress bar or blank screen several times.
If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model. Software Update shows only software that is compatible with your Mac.
Hope this helps. Take care.