I have only one bug left

My authentication box doesn't pop up sometimes. Did a full system check including a half hour memory test. Passed.

I use Alt+F2+r to refresh desktop and it fixes it.

9 year old Dell Optiplex 3010 w 8GBs. 3 yr old BIOS.

Rarely, my desktop icons would disappear. Had that on my old Lenovo also.

Cleaning the memory and slots with alcohol seemed to help.

Tried:

sudo apt-get install --reinstall nautilus

sudo apt install --reinstall policykit-1-gnome ca-certificates

It might be that Linux/Zorin is less tolerant of old memory.

Have you tried running Memtest86+ to see if RAM is failing?

Yes. I also ran the Dell 30 memory test.

Might yank out one stick and see what happens. 4GBs is plenty.