That's strange, I'm not sure why the URLs of your portfolio pages are constructed like that, but lets try something a little different!
Here is some code you can use to load a slideshow using a custom field:
<?php // Get the value of the slideshow custom field
$portfolio_slideshow = get_post_meta( $post->ID, 'slideshow', true);
// If it isn't empty, load a matching slideshow
if ( !empty( $portfolio_slideshow ) ) {
if ( function_exists( 'meteor_slideshow' ) ) { meteor_slideshow( $portfolio_slideshow, "" ); }
} ?>
You can add that to your portfolio template file, it will check for a slideshow custom field value and if there is one it will load a slideshow with that value as the slug. This might work better because you don't have to have a slideshow for each one if you don't want to, and the slideshows don't have to match the portfolio post slugs.
After you add the code, to add a slideshow, just create a custom field with the name "slideshow" and a value that matched the slug of the slideshow you want to load. Let me know how that works!