Jerky movement on onEnterFrame

I am using a typical onEnterFrame function to move a movieclip from right to left across the screen. (I have tried this using a simple _x coordinate shift as well as an easing algorithm).

In both cases, the movement is pretty smooth at high bandwidth speeds, but on slower download speeds, the movement is VERY jerky.

**_root.myMC.onEnterFrame= function(){

if(_root.myMC._x > 0){
_root.myMC -= 10;
}

};**

Has anyone a solution to this?