query_post() pagination issue

Hi
Working on custom post types.

I have a custom “query post” and it works but pagination fails it display the next and prev links but posts are the same as if it were the page 1.

I tried the “global $query_string” and the query stops working.

Heres the code for the template of the custom post type i am working in:

query_posts($query_string.
'&post_type=video&category_video='
.$category_video.'&posts_per_page='
.$videos_per_page.'&orderby='
.$order_by.'&order='.$order);

and then the loop

if( have_posts() ) : while ( have_posts() ) : the_post();
...
endwhile;
endif;

Please assists