From the codex:
Note: When using ‘some string’ to show
as a submenu of a menu page created by
a plugin, this item will become the
first submenu item, and replace the
location of the top level link. If
this isn’t desired, the plugin that
creates the menu page needs to set the
add_action priority for admin_menu to
9 or lower.
So on you add_action('admin_menu',array($this,'_wp_admin_menu')); set priority to 9 or lower like so:
add_action('admin_menu',array($this,'_wp_admin_menu'),2);
and you should be set.