Need help with a Vertical Scroll Loop

I have a list of locations that I am scrolling vertically in my movie with a mask over it (kind of like movie credits). My problem is when I try to loop the graphic it isn’t smooth. I am using the following script:

onClipEvent (enterFrame) {
if (_root._ymouse < 768) {
this._y --;
if (this._y < -149) {
this._y = 355;

It does work but, like I said it isn’t a smooth motion. I think I would need two of the same graphic and when one gets to a certain point on the stage the duplicate graphic should begin; then the first graphic should reposition itself back to the beginning and so on. If you have any suggestions I would greatly appreciate your help.