FCP 10.4 - Import problem

Help! I don't know if anyone's replicated this but it's a showstopper bug if there's not some simple workaround.

I am seeing slight but noticeable choppy playback on AVCHD footage imported through 10.4. The choppiness is NOT baked into the file - if I copy the imported footage into Premiere or through Finder, it plays back very smoothly. The choppiness appears in the browser, the timeline, and exports.

I've set the view to best performance, trashed renders and prefs, removed effects, even reinstalled FCPx altogether, and nothing seems to be correcting the playback issue. If I drop the footage into Motion, the problem is there as well. Any footage I imported before 10.4 is fine. Footage that's not AVCHD plays back fine, including some imported Mavic Pro 4k footage. This has been my workflow for 6 plus years with the exact same footage and I don't know what I'm missing. Any thoughts?

On a late 2014 Retina 5k iMac (32 GB) and M295X video card (4 Gb). Camera is a Panasonic AF-100.

Incidentally, I found a kludgy but successful workaround. I had been transcoding the files in Compressor to another format altogether so I could work with them - FCPx only seems to be having a problem playing back and rendering AVCHD files. Out of curiosity, I opened up one of the clips and dropped in its transcoded equivalent. Bam. The playback problems disappeared on every instance of that clip, in the timeline and the browser, even when I undid the action. This might help with this particular workflow until Apple issues a fix.