Macromedia technique support has mentioned about this point. Macromedia suggests making a slight scaling to the bitmap would solve the problem.
Here is the experiment:
1. The pixel shift occurrs inconsistantly. The _x and _y might affect the side effect.
2. Scaling would probably solve the problem. But how much we need to down-size or up-size ? I start downsize 1 twip, and it solves many but not always. To downsize 0.05 pixels, we can not do it in the panel box. It can be done only by Action script. Sometimes I need to down-size 0.1 or even 1 pixel in large size bitmap.
3. Incidentally I ever created a movieClip, that I put at the top row in the movie below. For unknow reason, it shrink automatically. I tried in other bitmap and it seems to work without pixel shift. I dont know the reason and I could not find any difference between it and other movieClip that hold the bitmap.
Here is the fla, you can download and try to find the difference. If you found any difference or if you can create a similar one, please let me know.
download zipped fla