iMac won't complete shutting down or restarting process
[I had posted this before, but the only answer I got (to restart in Safe Mode) didn't solve the problem. So, I try again.]
Whenever I try to restart my iMac (late 2009), everything goes as expected: one by one, apps close; the screen turns dark, but then the cursor/arrow jumps to the upper left corner of the screen, and nothing happens. It's not frozen, since I'm able to move the cursor. After that, the only thing to do is to force shut down.
I've just reinstalled OS 10.13.6, hoping to solve this, but it hasn't worked. There's a security update to be installed which won't, because the restart/install process never happens.
Also, I can't schedule the iMac to shut down and restart as I used to.
Any suggestions?
Thanks in advance!
iMac, macOS 10.13