[Lynne]

Create a new style with no parent - Styles & Templates > Style Manager > Add New Style > no parent - then browse the site using that totally default vbulletin style - do you still have the same problem?

If it is still happening, then try disabling your plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true);