My brand new iMac suddenly has a transparent grey bar that runs across the entire screen after being on a few minutes

Not sure its related to update to Catalina OS, but my brand new iMac suddenly has a transparent grey bar that runs across the entire screen after being on a few minutes. It disappears after a reboot, but then comes back on. Looks like the screen flickers as well. ???

[Re-Titled by Moderator]