When I import a NTSC video that has a 3:2 pulldown applied and put this video into a 23,976fps timeline, FPCX will auto remove the pulldown. This is really nice. But it will introduce a jitter, like it displays each field, but not the actual progressive frame. This may not a major problem on 4:3 content, but on any aspect ratio higher, e.g. 16:9, it's very obvious.
Does anyone may have an idea how to fix the jitter problem?
I think this was discussed in this thread on the Apple FCPX forum. It indicates pulldown removal in FCPX only works during ingest from a tape-based camcorder. To remove the pulldown after ingest you must use Compressor and pick the reverse telecine filter:
In FCPX you could also try creating a 720p progressive project, add the NTSC clip and select the deinterlace filter in the FCPX Inspector, render/export that and evaluate. For older 29.97 interlaced material I have from a Panasonic DVX-100p that seems to work well. However that material did not incorporate Panasonic's "advanced pulldown" which happens when you shoot at 24p and it records it interlaced. We always avoided 24p on the DVX-100.
Trying matching back to the clip, and moving the in point a frame forward or back, and then re-cutting in the clip. You may have to go two frames forward, depending. I've found that clips with pulldown will usually go into a 23.976 timeline correctly if the in-point is on a correct A frame. So if you get the cadence right, it should slot in properly.
The other way as joema mentioned is to pre-process the clip through compressor and do a reverse telecine to remove the pulldown.
For the reverse telecine option to appear (under the Quality sections, the menu for 'retime quality'), you have to specify "Progressive" for field order. Make sure you're also converting to 23.976 (or 24 if you're doing that). This is all under the Video tab, after you apply compression settings.
Your original footage with pulldown needs to be 29.97 with top or bottom field order, and it needs to have actual 3:2 pulldown on it, so make sure Compressor is interpreting it right also. You can change how the footage is interpreted by clicking on the name of the clip in the job window, but if your clip has correct metadata, it should automatically know this.