Why does Final Cut Pro X hate my BM video files?

I've been having a very annoying problem where I edit Apple Pro Res HQ files in FCPX shot on my Pocket camera, but after I share the video playback is choppy (stuttering on certain frames, generally trudging play). This choppiness still occurs if I convert the .mov file to MP4, and if I upload it straight to YouTube. The only time it didn't occur is when I uploaded to YouTube at a very low resolution. I experimented with editing on iMovie, and when I exported to YouTube it only chose files recorded with my gh4, completely ignoring the BM pocket files. Anybody know why this problem is occurring, and/or what plug-ins I might need with FCP to make the problem go away?

I'm editing on a late 2015 iMac:

Mac OS Sierra Version 10.12.6
Retina 4K

GPU: Intel Iris Pro Graphics 6200 1536 MB

Processor: 3.3 GHz Intel Core i7

I’ve edited bmpcc prores HQ files on fcpx on iMacs hundreds of times and never had the problem you are describing. I’d look at a possible corrupted sd card or the camera itself.

It's possibly a frame rate issue.
Check your sharing settings in FCP and make sure the frame rate matches your timeline.
If you can post a link to a video example it may be easier to see what the problem is.

It's also VERY unlikely that the BMD source files are an issue so you may want to try FCP forums for solutions.