What are the potential causes for a sudden MySQL slowdown?
You could try the following to improve things

skip-host-cache and skip-name-resolve

Please follow these steps:

Step 01) Add the following to /etc/my.cnf

[mysqld]
skip-host-cache
skip-name-resolve


Step 02) service mysql restart

Step 03) Stop using mysql users that have DNS names. Run thus query:

SELECT user,host FROM mysql.user;


Any user that has a DNS name, simply replace it with IP Address (such as 10.85.115.73) or a netblock (such as 10.85.115.%)

You may also want to run this command intermittently (say one an hour in a crontab)

FLUSH HOSTS;


Give it a Try !!!