Slave receives end packet from master

[Database System]

Hi,

We have been using replication (MySQL5.0) for several years and no problem. Recently we
use new servers with new OS, and new MySQL 5.1. 

The replication keeps on and off (Slave I/O thread value changes between Yes and No
automatically). It comes back on its own, but since it being off so often, and doesn't
have enough time to catch up, it is behind master. The error log says:

110720 11:13:22 [Note] Slave: received end packet from server, apparent master shutdown: 
110720 11:13:22 [Note] Slave I/O thread: Failed reading log event, reconnecting to retry,
log 'binlog.000084' at postion 47000547

The master is on, running fine.
When the slave I/O thread value is NO, I did ping from slave to master, and it was no
problem.

What the problem it could be?

Thanks,
Lisa