Random Adsense Block and Drupal Cache

Hello Everyone.

Now I'm trying to set Random Blocks in my Drupal Site.

The code is very simple and it's added in the page.tpl.php.

Something like that:

<?
$variable_random = rand(1,2);
if ($variable_random == 1)
//Show Adsense Block #1
else
//Show Adsense Block #2
?>

So far so good, but this only works when Caching mode: is Disabled, because Drupal loads the page.tpl.php every time that the pages is loaded.

So my question: is there any way to do this using the Caching mode: in Normal Option?

I would appreciate any help.