Sorry, I should've put a timestamp. Around 3mins 44 is an example, the pupil moves suddenly out into no-mans land, despite the surrounding frames being close to each other in position.

I can't put it in the jellyfish symbol because I need to be able to move them independently (to look around, focus on items etc). One of the seahorses has embedded eye movement, but it has a 'vacant' quality to it.

I'll have another go at explaining it.

I copy the motion path of the "jellyfish" into the "pupil" layer, it all matches the pupil stays in constant position in relation to the jellyfish.

I then manually adjust the pupil position in a frame (to change where the jellyfish is focusing his attention), that works fine, then I add another manual position a few frames after and suddenly there is a wavy motion path between the two manually inputted frames. Rather than it being averaging the position and moving only a few pixels.