[Lynne]

Create a new style with no parent:

• Styles & Templates > Style Manager > Add New Style
• Parent Style: No Parent Style
• Title: Default vBulletin
• Allow User Selection: Yes
• Save

Then browse the site using that totally default vbulletin style - do you still have the same problem?

If you really have updated all your templates all the time, then the default style should also have the problem. And if so, try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true);