Mirage/Reflection Effect

I'm trying to create a background wobble to represent moving water. I have the Persective Object using a Sinewave at the moment that mvoes left and right gradually and works okay... Until you start scrolling the screen. Is there any way I can prevent the Perspective Object from warping rapidly during scrolling, or is there another way I can achive this effect?

Example of what's wrong: Please login to see this media element.

Thanks for the info, I'm trying not to add too many extensions to the list, but this does work really well. I managed to get the desired effect with the Perspective Object, I jsut needed to set the object to the width of the frame and adjust the Offset and now it's working as I wanted.