I have discovered that when I edit a page in IE8, although the page uses the default template, the item selected in the drop down menu under Page Attributes is always the second template on the list. This means that every time a page which uses the default template is edited, I have to remember to reselect the template or the navigation disappears. I notice that none of the options are ‘selected’ in the code.
I thought Firefox was OK but there seems to be a similar issue with Firefox, though not so easy to reproduce.