out of memory

Drupal is known to be resource heavy system that heavily depends on database storage for content generation. While minimum recommendation for PHP memory limit is 32MB, it is not uncommon to allow as much as 128MB of RAM to PHP in bigger sites.

With tons of modules installed, Drupal cron job can fail due to lack of memory even with 128MB. Quick solution is to increase PHP memory limit. There are several ways to do it and they are well explained at http://drupal.org/node/207036.