Occasional Drupal Error in bootstrap.inc
The bootstrap.inc file is included, from index.php, using require_once './includes/bootstrap.inc' (Drupal 6) or require_once DRUPAL_ROOT . '/includes/bootstrap.inc' (Drupal 7). That one of the first instruction executed from Drupal when bootstrapping, as the bootstrap code is all contained in that file.

The errors you are seeing could be caused from a module that is using include or require to load the bootstrap.inc file, which would not be necessary, as the file is already loaded from Drupal.
Alternatively, the errors are caused from a module that defines those constants in one of its files, which is then causing those errors once it is loaded. For example, it could be a module that implements an alternative to the cache mechanism implemented from Drupal, which defines those constants with the wrong assumption that they would not be available.