I have glitchy playback as well when using this technique. During playback it will play back 10 or so frames smoothly when arrowing through it, and then one frame will pop up that is from a few frames back, and then the next frame is the next in order again. And the cycle continues.
This is on a rendered movie mind you. I’ve brought in the movie back to FCP both before and after the conform from 60p to 23.98 and it does this no matter what… within QT or FCP.
Thought I might be trying to go too slow with it but even a 40% reduction using optical flow caused this.
FYI my video was from a go pro where the shutter speed can’t be set manually. But regardless, this shouldn’t happen, the effect might not be as convincing but it shouldn’t be staggering frames like that.