Hmm im not sure what's happening here because I've never used that method for inserting flash in SS as yet.

For what it's worth I would usually handle all my flash work in the template but if it must be dynamic then I set up the flash embedding code in the template and use a CMS field to pass on the URL of the SWF.
So in the page definition file I might create a field called SWFLocation and add it to the CMS fields:

public static $db = array(
   'SWFLocation' => 'Text'
   );
   
   function getCMSFields() {
   $fields = parent::getCMSFields();
   
   $fields->addFieldToTab('Root.Content.Main', new TextField('SWFLocation'), 'Content');
           
   return $fields;
   }

Then in your template for that page you can set the path in your flash embed code to something like:

http://www.yoursite.com/assets/$SWFLocation

This way all you have to do is add the name of the file in the CMS field. Hope this helps a bit.