I'm using CS3 and I'm getting some wierd movement occasionally between keyframes in "position" and it's not the boomerang effect. The frames aren't identical, but the z position SHOULD be the same and not change (the x and/or y do). I've looked at every angle I can find and used the graph editor and can't figure it out. In the preview screen I can't see the movement in the path but when I use the graph editor and use the value view I can see the odd random jerky movement in the z line. It's almost like a wiggle (which I've never actually used) and not a smooth continuous motion like the boomerang effect. :-/
I can't find anything on google about it, although I might just not know the write phrasing. Does anyone know what might be causing this??? It's driving me crazy!
Without screenshots, system info and details about your project nobody can know. That said, quite often AE's crappy graph editor will draw jittery lines when zoomed in due to how it iteratively quantizes the curve drawing and there is actually nothing going on. So unless this is an issue with final rendering, I wouldn't go crazy over it.
I'll bet that if you look closely at the motion path in the Comp window you'll see bezier handles. Press the G key until you get the convert point tool and click on the vertices that are giving you problems or adjust the bezier handles to fix the problem.