Movie clip movement help!

SO, i have a movieclip that slides back and forth depending on which way the mouse moves but I need a dead space in the middle of the 800px wide stage so the clip doesn’t keep scrolling when I am in the middle so I can click on navigation items… here is the script I am using to make it work… anyone know how to give it a dead space of like 300 px in the middle of 800 px stage?

onClipEvent (load)
{
xcenter=400;
speed=.75/10;

}
onClipEvent (enterFrame)

{
if (_global.menus)
var distance=_root._xmouse-xcenter;
_x+=(distance*speed);
if (_x > 0) _x=0;
if (_x < -870) _x=-870;
}

any help would be much appreciated… Thanks in advance…