We tried increasing the memory yesterday adding define( 'WP_MEMORY_LIMIT', '64M' );
That's a good idea. As you've probably discovered, this error is a common WordPress issue:
http://codex.wordpress.org/Common_WordPress_Errors#Allowed_memory_size_exhausted
If it keeps happening, I would suggest that you get in touch with your hosting provider, as the constant you've added to your wp-config.php might get ignored if there is no available memory for your site.