I was cutting a quick video—which ended up not being so quick—for a friend. One not-so-small clip was recorded in 1080i 60 fps. Once imported into FCPX there were these twitchy artifacts forming every couple seconds during playback.
I searched the web and the problem has to do with needing to first de-interlace the source video. One solution was to download a program called JES Deinterlacer. After a number of failures—the program would freeze after processing a short number of frames—I gave up and went to bed.
The following morning I suspected that the freezing problem was due to the fact that I was working off an external hard drive. No bother. So after copying the file over to my Desktop I was able to process the entire video. The resulting file, after importing into FCPX, was free of those twitching artifacts.
Sadly, however, the quality was degraded. But that is a side effect of de-interlacing. Read more about it here.
Also, one solution was to allow FCPX to do the do-interlacing when you share out a master file, but I had no luck with that. Any time I rendered the project the resulting master file would contain the artifacts.
I am very thankful for the JES Deinterlacer.