Regular session table crashes, on a large multi domain, multi site Drupal set-up
I would convert the user &amp; session tables to InnoDB. I would also check the memory limits for MySQL. It's not the most ideal tool, but if you're in a shared environment you can install a copy of Drupal Tweaks (http://drupal.org/project/drupal_tweaks) and see if you are able to alter any of the MySQL configuration directive such as memory limit.