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.
I hope this points you in the right direction or at least helps to shed some more light on the cause of the problem.
SimpleViewer Support Team.