I have taken a look at Monica's WD7P file. After exporting the site and placing it on my two local web servers, one Windows and one Linux and viewing the pages with multiple browsers it seems the ecwid shopping bag is the code causing the pages to jiggle on first load.

Using the Opera browser the ecwid logo is seen to the left of the page as it moves to the left. A fraction of a second later when the logo disappears the page moves back to the right. Looking at the pages that jiggle there is code for the shopping bag or code to add to the shopping bag. Pages without that code do not jiggle on first viewing.