Animation Speeds Up Each Time 'start' Button Clicked?

one could add a 'start' & 'stop' (id="stopit") button to control the simple sprite animation. My Javascript looks like this (note that it works in tandem with the core.js that comes with the book; see the ZIP file i've attached):

Code:
var Robot =
{
init: function() // initiate via core.js
{

[Code]....

clicking on the 'start' button more than once speeds up the animation, more with each successive click. Clicking the 'stop' button will only finally stop the animation if it's done the same number of times as the 'start' button was clicked.