Add the height to the loadarea (addition highlighted):
Without that, the loadarea constantly becomes 0px high in between each change. That's what makes the page jump.
<div id="loadarea" style="
height: 417px; width: 400px;"><img src=http://prettysmartcart.com/demo/images/RazorbackTankGrayLG.jpg></div>
I just picked 417px as that's the box dimension height of loadarea with its initial content in it. If there's other higher content, use a value that will accommodate that height. In a layout like that page has, ideally all content should be the same height and width and the dimensions of the loadarea should be set to that. If not, then the dimensions of the loadarea should be set to the highest height and the widest width found among the various potential content blocks. These might even be two different ones, one for width, another for height.