Hi Moonj.,
I've tried as many of your suggestions as possible. In safe mode, Chrome goes all wonky, as well as other other programs, making it pretty much unusable. Not sure what that means.
I've tried the following:
•reinstalled OS (10.13.6) yet again, along with latest security update
•uninstalled Flash, as I thought maybe that was causing the problem and it's going away in December anyway
•uninstalled Perian but it still shows up in System Preferences to allow a reinstall
•the only other program that shows up in system Preferences is Flip4Mac, which is still active
•checked for login items, with only iTunesHelper and Dropbox being there and active
The problem persists! I think the problem with Mail may be a symptom, not a cause. Once the problem occurs, Mail fails to delete items or download new items of mail. Also, I am then unable to use the "restart" command under the Apple icon at top left of screen and must do a force-quit and then restart. Once I've done that, everything is OK for a while.
This morning, the only program I had running after turning on the computer was Mail, which bombed out after deleting and/or reading maybe 40 emails. Once it bombs out, I can only force-quit it, but then it will no longer activate and the problem continues until I force-quit and restart the computer.
Any further suggestions? I really don't want to lose my 32 bit capabilities by upgrading to 10.15, but I'm at a loss as to what else to do.
Thanks for your help so far!