Undefined variable: node
If you are using drupal 7, there should already be a node object available when you are looking at a full node page. Looking at http://api.drupal.org/api/drupal/includes--theme.inc/function/template_preprocess_page/7 - the default implementation embeds it into the $variables array:

  if ($node = menu_get_object()) {
    $variables['node'] = $node;
  }


If your theme is overriding this, you can simply add it back in.

An example might be:

function YOURTHEME_preprocess_page(&amp;$variables) {
  if ($node = menu_get_object()) {
    $variables['node'] = $node;
  }
}