Loop Problem. Works in PHP4 not work in PHP5

Hello,

I have a little modification on the index template that works fine unit i switch to php5.

the code:

<?php $my_query = new WP_Query('cat=789&showposts=10'); ?>
<?php while ($my_query->have_posts()) : $my_query->the_post(); ?>
## Some html and echo
<?php endwhile;?>

<?php query_posts('cat=-789&showposts=10'); ?>
<?php while ( have_posts() ) : the_post() ?>
## Some Html and echo
<?php endwhile;?>

The problem:
The systen not change the content when i try to switch to older post. And it pick up some realy old post.

The code work very well with php4 (4.4.7 i think) but not with php 5.2.6