Display Random Articles With Thumbnails in Posts

Hello,

I have tried to display at the bottom of my post random articles and thumbnails (this are from custom fields) but with no success.

I have used this code:

<?php $my_query = new WP_Query('orderby=rand&showposts=4');
while ($my_query->have_posts()) : $my_query->the_post();
?>

<li><a href="<?php the_permalink() ?>"><img style="border: none;" alt="random article" src="<?php bloginfo('stylesheet_directory'); ?>/timthumb.php?src=<?php echo get_post_meta($post->ID, "thumb", true); ?>&amp;h=47&amp;w=140&amp;zc=1;" /></a>

The good news it that it generates random articles with thumbnails in my post, but the bad news it that it random the comments from my blog also.

Any help will be much appreciated !

Thank you.