Start by disabling all mods/plugins and testing on the default vbulletin skins. If that works re-test on your new skin, the mods/plugins should still be disabled. Once you have done this report back with the results.
To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php
Code:
define('DISABLE_HOOKS', true);
You can also shut off the plugins from the admincp:
ACP --> settings --> options --> Enable Plugin/Hook System: NO