Hi guys,
I had exactly the same problem with glitches (that have been also exporting to video files rendered in fcpx) on both GTX 680 and GTX 1060 cards. Tried going from default os x video drive to web driver, no changes. The correct solution was to remove InjectIntel=True from clover config and to set in BIOS to initialize first PCIE card and disabled intel graphics. Boom! Since that moment all problems were gone. Also rendering times in FCPX improved enormously. When glitches were present, it took more than 8 hours to render a 4 minute 4K video with a lot of color grading, and right now I'm seeing that it took only 7 minutes to have 25% of the job done. So to summarize, to get rid of the glitches you need or to completely disable built-in graphics, or to (my guess) inject it correctly.
PS: OS X version is High Sierra 10.13.2.