Due to some reason the default Ubuntu distribution is shipping with only root permissions on /var/cache/bind while named is running as the user bind. The named process is trying to write to the /var/cache/bind directory however fails due to the fact that this can only be done by the root user. This in turn results in retries which take a lot of your CPU away from other processes.
To resolve this, one of, the solution is to reset the permissions on /var/cache/bind which can be done with the commands below;
chown root:bind /var/cache/bind chmod g+w /var/cache/bind
After this you will have to stop and start bind which can be done with the below command.
service bind9 restart
This should result in a lot less CPU usage from the named process.