intermittent vertical split screen & flickering display

How do I correct and stop intermittent vertical split screen and display flickering on new to me (10days) refurbished iMac (Retina 5K, 27-inch, 2020). After 6 days, using "Numbers" app the screen just split in two vertically with identical information in both screens. Later the same day a U-Tube video bounced back and forth between vertical split screen and flickering. couple of days later in "Safari at a genealogy site the plot screen and flickering came back. I tried to get a video on my iPhone each time. At the time I was not aware of the green full screen button options and I did not use it.

After reading related Community results ran the diagnostic check and the app "Found nothing wrong". I have been operating a macOS High Sierra iMac from this same setup successfully for 11 years.

Can this be fixed in settings or should I send the iMac Back? back?