when working with images you should keep aliassing under control.
Check that all coordinates are cool (everything should always move by 1 px at a time, and not go into floating points...because the reandering engine rounds them).

In the authoring environment check the coordinates of all the interested objects and if you need to, set the floating cifres to 0.
In the scripts just add this to your coordinates:

from

myMC._x = k;
myMC._x = Math.floor(k);

This should solve the problem..

Cheers