I have some files with some scatter points animated to move in constant velocity. However, the movie shows relative motion of the scatter points making the animation appear jerky.
The reason the scatter points look like they are moving relative to one another has to do with the way they are drawn on the screen. Any object drawn to the screen will be rounded off to the nearest pixel. The movement between scatter points is an artifact of rounding off the values to the nearest pixel. Sometimes one scatter will round up and another round down while other times both will round the same direction. The result is an occasional variation of one pixel in the distance between any two scatter symbols. The result in the animation is the relative motion effect you see. You can fix this by creating an antialiased animation. To do this in the AVI Exporter dialog place a checkmark in the Antialiasing checkbox and then set the Supersample Factor to 5 (or higher). This will make the animation look right but at the cost of processing time. Antialiasing solves this problem because it makes it so objects are not required to fall on a whole pixel. If an object falls on a fractional pixel each of the two pixels near it is shaded in proportion to that fraction. The factor controls the fraction size so 3 is about 1/3 of a pixel. This blends the image (some might say blurs) and creates a smoother effect that your eye does not pick up so readily.