Scan line when scrolling image

I am scrolling an image and I can see a horizontal scan line when it move.

I tried a lot of thing to solve this (recreated the image as a bitmap data, slicing the image in smaller pieces, making the bitmap at 99% scale inside the movieclip) but the scan line is always there.

I tried on IE, Firefox and Chrome and the problem is there, but it seems to be ok on Mac safari.

Anyone know how to correct this.

Just did a simple example of the problem:

Thx in advance