Thanks again, Aravisian. You're a gold mine of information.
It's possible that I have suppressed the rtkit notifications, following advice from this thread:
In summary, adding LogLevelMax=3 to /usr/lib/systemd/system/rtkit-daemon.service:
[Service]
ExecStart=/usr/libexec/rtkit-daemon
Type=dbus
BusName=org.freedesktop.RealtimeKit1
NotifyAccess=main
CapabilityBoundingSet=CAP_SYS_NICE CAP_DAC_READ_SEARCH CAP_SYS_CHROOT CAP_SETGID CAP_SETUID
PrivateNetwork=yes
LogLevelMax=3 <<<<<<<<
Since I'm still learning step by step, I'm sure you or someone else may throw up your hands in horror and cry "Nooo!!!", but I'm open to better suggestions. At this moment, though, it looks as if I have a workaround, if not a permanent solution. The Zorin main menu has been displayed for the past 10 minutes instead of disappearing several times a minute.
I want to thank you people for all your support. I would never have found this solution (I hope... 
) without your help.