Tiny Box JS removed from the page soon after flushing the page cache

TinyBox does not work for anonymous users.
When I flush the cache for the front page, where it shows up several time, I sometime succeed at getting anonymous users seeing the tinybox splashing correctly for some time (10-30 minutes)
Then, no splashing occurs anymore. Looking at the code page, both the js script and the inline code are absent.
The code is rather simple :

function tinybox_init() {
  // Necessary to check if we're on the frontpage
  drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
  $tinybox_content = "";
  $tinybox_splash_source_value = variable_get('tinybox_splash_source_value', '');
  $tinybox_front = variable_get('tinybox_front', 0);
  if ($tinybox_splash_source_value && ((drupal_is_front_page() && $tinybox_front == 1) || $tinybox_front == 0)) {
    $tinybox_splash_source_type = variable_get('tinybox_splash_source_type', 'nid');
    $tinybox_splash_delay = variable_get('tinybox_splash_delay', 0);
   
    if ($tinybox_splash_delay) {
      if (!isset($_SESSION['tinybox_splash_session_time'])) {
        $_SESSION['tinybox_splash_session_time'] = time();  
      }
      
      if ((time() - $_SESSION['tinybox_splash_session_time']) <= $tinybox_splash_delay) {
        return;
      }
      else {
        $_SESSION['tinybox_splash_session_time'] = time();
      }
    }

    $tinybox_splash_autohide = variable_get('tinybox_splash_autohide', 0);
    $tinybox_splash_css = variable_get('tinybox_splash_css', '');
    $tinybox_splash_css = ($tinybox_splash_css == '') ? drupal_get_path('module', 'tinybox') : $tinybox_splash_css;
    $tinybox_front = variable_get('tinybox_front', 0);
    drupal_add_js('sites/all/libraries/tinybox/tinybox.js', 'module');
    drupal_add_css($tinybox_splash_css .'/tinybox.css');
    
    if ($tinybox_splash_source_type == 'nid') {
      $node = node_load($tinybox_splash_source_value);
      $tinybox_content = str_replace(chr(13), '', $node->body);
      $tinybox_content = str_replace(chr(10), '', $tinybox_content);   
    }
    
    if ($tinybox_splash_source_type == 'content_type') {
      $sql = "SELECT nid FROM {node} n WHERE n.type = '%s' AND n.status=1 ORDER BY n.created DESC";
      $results = db_query_range($sql, $tinybox_splash_source_value, 0, 1);
      $data = db_fetch_object($results);
      
      $node = node_load($data->nid);
      $tinybox_content = str_replace(chr(13), '', $node->body);
      $tinybox_content = str_replace(chr(10), '', $tinybox_content);   
    }
   
    if ($tinybox_splash_source_type == 'views') {
      if (module_exists('views')) {
        $view = views_get_view($tinybox_splash_source_value);
        if ($view) {
          $view->preview('default');
          if (count($view->result)) {
            $tinybox_content = $view->render();  
          }
        }
        else {
          $tinybox_content = 'Views not found: ' . $tinybox_splash_source_value; 
        }
      }
      else {
        $tinybox_content = t('Require Views module.');
      }
      
      $tinybox_content = str_replace(chr(13), '', $tinybox_content);
      $tinybox_content = str_replace(chr(10), '', $tinybox_content);   
    }
    
    $tinybox_splash_width  = variable_get('tinybox_splash_width', '');
    $tinybox_splash_height = variable_get('tinybox_splash_height', '');
    
    $tinybox_splash_width  = ($tinybox_splash_width == '') ? 0 : $tinybox_splash_width;
    $tinybox_splash_height = ($tinybox_splash_width == '') ? 0 : $tinybox_splash_height;

    if (!empty($tinybox_content)) {//display Tinybox only if some content is present
      drupal_add_js('
        window.onload = function() {
          TINY.box.show({
            html: \'' . $tinybox_content . '\',
            boxid:\'tbox\',width:' . $tinybox_splash_width . ',height:' . $tinybox_splash_height . ',fixed:false,
            maskid:\'blackmask\',maskopacity:40,
            autohide:' . $tinybox_splash_autohide . ',
            closejs:function(){closeJS()}
          });
          document.getElementsByClassName(\'tmask\')[0].onclick=null;
        } 
      ', 'inline');
    }
  }
}

Thanks for help