Horizontal scroll hiccup problem

Hi guys,
Im new here and i have my first question. I need really your help.
I read every single tutorial and i didnt found the right answer. Could you please help me?

May be you are exhausted to here about the famous horizontal scrollbar, but i have really a problem.
My horizontal scroll looks like its having several hiccup problems :-/.

I have a movie clip, how have this code:

onClipEvent (load)
{
xcenter=150;
speed=1/10;
}
onClipEvent (enterFrame)
{
var distance=_root._xmouse-xcenter;
_x+=(distance*speed);
if (_x > 0) _x=-300;
if (_x < -300) _x=0;
}

And all the scene looks like in the pic i attached.
The pink rectangle is just a mask and the gray buttons are compiled in a movie colled “all in 1”.

So the structure (_root) looks like:
Scene 1/all in 1/20 btns_mc are inside
The code is pasted under:
Scene 1/all in 1 (where the all buttons are compiled)

But why is my horizontal scroll having several hiccup problems?
A! to see the hiccup Problem please put your mouse in the half of the scene and the end of the mask you will see the problem.

Thank you very much for your advice.

Cheers

sximmer

P.S. I also duplicate the “all in 1 movie” on the right side but nothing happened.

EDIT:

Please guys i need really your help. I expend 3 Days almost just trying to solve the problem.

I will really appreciate your help.
At the end i will share the fla for all the other guys how need a horizontal scrollbar. :thumb2:

Cheers