Actionscript problem

I have this code to make a text go from slow to faster and faster:

onEnterFrame = function () {
ypos = ypos + 0.1;
MyGraphic_mc._y = MyGraphic_mc._y - ypos;

The problem I have is, when the Scene loops (starts from the beginning) it just keep go faster and faster. I would like it to stop when the Scene go loops and start from the beginning, slow then faster faster again.

Hope someone understand my problem