Footage that looked smooth on the camera can play with a recurring hitch once it is on a timeline. The cause is arithmetic: the source rate and the project rate do not divide into each other cleanly.
Conversion happens by repeating or discarding frames
Playing material at a different rate to the one it was shot at requires either inventing frames or removing them, because the count per second must match the timeline.
The simplest method duplicates or drops whole frames on a repeating schedule, and that schedule produces an irregularity in motion that recurs predictably.
Human vision is highly sensitive to regular irregularity, so the pattern is far more noticeable than random unevenness of the same magnitude.
Clean multiples hide the problem
Where one rate divides evenly into the other, each source frame is held for the same number of output frames, and the cadence is uniform.
Uniform cadence reads as slightly stepped motion rather than as stutter, which the eye accepts because nothing about it changes over time.
Rates that share no clean relationship produce an uneven repeat pattern, and that unevenness is what viewers describe as juddering.
The fractional rates are the usual culprits
Several broadcast frame rates are not whole numbers, sitting fractionally below their nominal value for historical reasons connected to colour television.
Mixing one of those with its whole-number equivalent produces a drift of one frame every few seconds, which appears as a periodic hitch and as progressive audio sync loss.
Because the drift is slow, the fault is often missed on short review clips and only becomes obvious across a full-length programme.
Shutter angle makes it worse or better
The amount of motion blur in each frame depends on shutter speed, and blur helps the eye accept imperfect cadence by softening the transition between frames.
Footage shot with a very fast shutter has crisp frames with no blur, so every repeated frame is clearly visible and stutter is exaggerated.
This is why the same conversion looks acceptable on one clip and unusable on another shot in brighter conditions at a higher shutter speed.
Optical flow is a different trade
Motion-estimating conversion generates new intermediate frames rather than repeating existing ones, which removes cadence irregularity entirely.
The cost is that generated frames are guesses, and they fail visibly where objects cross or where fine detail moves quickly, producing warping artefacts.
The reliable answer remains matching rates at acquisition, since every conversion method is choosing which kind of damage to accept rather than avoiding damage.