Thanks for the sharing of your time and expertise, Robin.
I had never before modified field dominance settings, so I researched how to do so in FCPX and tried the available pop-down options— no improvement using any of them. All my footage is 1080p, as an aside.
Then I began exploring frame rates, again thanks to your feedback. I discovered all my footage is at 59.94, but my project was set at 23.98! YES! A eureka moment— I thought. I tried to increase the frame rate of the existing project but the frame rate pop-down was grayed out. So, I created a new empty project with the desired frame rate of 59.94 and I copied and pasted the entire contents of my problem project into the new project. I WAS ABSOLUTELY CONVINCED this was going to make life beautiful again, but nope— that same visible nastiness remains.
In this newly created project I again tried the available field dominance settings for the heck of it, but again without any change.
What in the world could this be, Robin? Anyone?