If you’re having problems with Drupal cron — like cron runs are timing out or running out of memory — here’s a quick hack to figure out which module is causing the problems.
Edit the includes/module.inc file, and find the function module_invoke_all. Insert the line indicated below to have a notice added to the logs (admin/logs/watchdog) for each module cron is running. You can then figure out the last module running when the problems occur.