Unfortunately, I do not know what is causing your menu elements to separate momentarily whilst switching pages.
However, here are a few things to try/consider:
(1) As your web page is essentially two tables nested within each other, please double-check that the widths and heights of all your table elements (rows and columns) add up correctly.
(2) Try setting a CSS max-width on your menu container.
(3) Try temporarily replacing you gallery's embedding code with just an empty <div> with the same dimensions as your gallery to see if the problem persists:
<div id="empty" style="width: 1024px; height: 609px;"></div>
(4) Try embedding the SimpleViewer gallery outside the nested tables on your web page.
(5) You could perhaps try laying out your web page using CSS instead of tables.
(6) Instead of having your menu duplicated on every one of your web pages and loading a new page each time a menu item is clicked, you could perhaps keep everything on a single web page and load the chosen content dynamically into a container on the web page (depending on which menu item was selected) using JavaScript. The user would always stay on the same page and the menu would never have to be redrawn after initially being displayed.
I hope this points you in the right direction or at least helps to shed some more light on the cause of the problem.
Steven Speirs
SimpleViewer Support Team