How do I make a stable sidescrolling background

Im making a sidescrolling background, like a treadmill, you it seems like you are always moving, It goes like this

drive.g

oogle.com/file/d/0B5Aa0lJtvsntMlcyOFV3SS1jR2s/view?usp=sharing

If you go to preview you will notice that every X time they get too close, and then too far, how can I fix this?

Is there any specific reason you are not doing it the way as shown in the autorunner example?

The reason it's not precise is because the position isn't precise before you change it..

I did not knew about this autorunner example, where can I find this wonder?

found it, ty