$_GET['q'] on homepage is undefined

We're experiencing the problem on all Drupal 6 platforms which have Droptor installed, so this does not seem to be a fringe case. It only happens on the homepage, where $_GET['q'] appears not to be set at this point in the execution process.

Perhaps it is due to the following (from bootstrap.inc):

      // If there is a cached page, display it.
      if (is_object($cache)) {
        header('X-Drupal-Cache: HIT');
        drupal_page_cache_header($cache);
        // If the skipping of the bootstrap hooks is not enforced, call hook_exit.
        if ($cache_mode != CACHE_AGGRESSIVE) {
          bootstrap_invoke_all('exit');
        }
        // We are done.
        exit;
      }

I think this is important as it is called in the sixth bootstrap phase, where as $_GET['q'] is set in the eighth phase. So the variable is not as yet set. But it only seems to affect the homepage (?).

It is an unusual condition as it causes errors on every homepage load on one platform but on every third load on another.