You need to create random.php page in your template.  In that page just copy paste the following code:

<?php
query_posts(array(‘orderby’ => ‘rand’, ‘showposts’ => 1));
if (have_posts()) :
while (have_posts()) : the_post(); ?>

<h1><a href=”<?php the_permalink() ?>”><?php the_title(); ?></a></h1>

<?php the_content(); ?>

<?php endwhile;
endif; ?>

Than crate page through wordpress admin panel and select the random page themplate.

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