next_posts_link() links to the same posts, not to the new ones.

Hi.
Thanks to your support, and the codex, I came to this

<?php
/*
Template Name: index
*/
?>

<?php get_header(); ?>

<div id="inhalt" class="text_single">

<?php
// set the "paged" parameter (use 'page' if the query is on a static front page)
$paged = ( get_query_var( 'paged' ) ) ? get_query_var( 'paged' ) : 0;

// the query
$wp_query = new WP_Query( 'cat=3,9,10,23&paged=' . $paged );
?>

<?php if ( $wp_query->have_posts() ) : ?>

<?php
// the loop
while ( $wp_query->have_posts() ) : $wp_query->the_post();
?>
<?php the_title(); ?>
<?php the_content(); ?>
<?php endwhile; ?>

<?php
// usage with max_num_pages
next_posts_link( 'Older Entries', $wp_query->max_num_pages );
previous_posts_link( 'Newer Entries' );
?>

<?php
// clean up after our query
wp_reset_postdata(); ?>
<?php else:  ?>
<p><?php _e( 'Sorry, no posts matched your criteria.' ); ?></p>
<?php endif; ?>

</div>

<?php get_sidebar(); ?>
<?php get_footer(); ?>

And the posts are shown. But if click on the “Older Entries” link, the URL is …/page/2/ and the posts are just the same.
I just found the code here http://codex.wordpress.org/Function_Reference/next_posts_link and just put it between the header and footer stuff. So I’m wondering why it’s not working. I read it’s better to change $the_query to $wp_query, so I did that. But it didn’t change anything. I’d greatly appreciate if someone could give me a hint.