Slave I/O thread dies very often
When the IO Thread dies, it usually dies for these reasons


STOP SLAVE;
STOP SLAVE IO_THREAD;
Network Issues


The first two reasons are obvious, but many have been victimized by network connectivity. For example, if there are enough dropped packets along the traceroute of the IP you are using as the Master, the IO Thread will just die off. By default, mysql attempts to reconnect to the Master every 60 seconds.

The mysqld process does not record network level issues, but a dying IO Thread is usually a dead giveaway (no pun intended). If this is happening often, go to work troubleshooting the traceroute on that IP (perhaps ifconfig on that Master IP).