Posts by zglows

After this piece of PHP code is added to my template (Wordpress) my server goes bananas. (I see lots of load peaks, it becomes unresponsive and the site won't load until I reboot the server.)
The code just displays random posts from the database:

<?php
$random_query = new WP_Query('posts_per_page=4&orderby=rand');
while($random_query->have_posts()) : $random_query->the_post(); global $post; ?>

	<li>
	<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><img src="http://mydomain.com/images/<?php echo get_post_meta($post->ID, 'thumbnail', true); ?>" alt="<?php the_title(); ?>" title="<?php the_title(); ?>" /></a>
	</li>

<?php endwhile; ?>
<?php wp_reset_postdata(); ?>

Does anyone know how can I optimize this piece of code to avoid having all these problems?