The flash could be due to the new page loading plus the almost instant revealing of a specific header due to your use of the URL parameter feature in each header's URL:
Code:
<div class="menuheaders"><a href="civil_design.htm?menuheaders=0" title="CSS">Civil Design</a></div>
This is just my theory, as in general the browser simply shouldn't be "flashing" when it renders a page. The issue could be a combination of your specific page's layout, coupled with IE's renderng engine.
What happens if you remove the URL parameter from the URL in your headers (ie: ?menuheaders=0)?
Bookmarks