[Lynne]

A few things...

Maintenance > Diagnostics > Suspect File Versions - no files should be listed as not containing expected results - is that the case with your site?

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); 


Try using a totally default style and see if you still have the same problem. To create a default style, 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

Set this as the default in AdminCP > Settings > Options > Styles & Languages > Set Default Style. Then go to Maintenance > Clear System Cache. Then browse the site using that totally default vbulletin style - do you still have the same problem?

Take a look at your error_logs (if you don't know where they are, ask your host) and see if anything is in there.