"Could not parse relay log event entry." error on slave

[David Griffiths]

We have a master-slave setup in production.

The master is running on a dual-Opteron with SuSE 8 SLES.

The slave is running on a dual Xeon with SuSE 9.

Both run MySQL 4.0.20

We recently moved our traffic database to the machine and started 
writing additional traffic (perhaps as much as 600,000 inserts/updates 
plus at least as many selects per day).

We use Nagios to monitor the machines, and have gotten alerts that the 
slave is not responding (this started yesterday, which is our busiest day).

This morning, the alert appeared again, but this time, there was an 
error in "show slave status"

"Could not parse relay log event entry. The possible reasons are: the 
master's binary log is corrupted (you can check this by running 
'mysqlbinlog' on the binary log), the slave's relay log is corrupted 
(you can check this by running 'mysqlbinlog' on the relay log), a 
network problem, or a bug in the master's or slave's MySQL code. If you 
want to check the master's binary log or slave's relay log, you will be 
able to know their names by issuing 'SHOW SLAVE STATUS' on this slave."

I am running a "mysqlbinlog" on the current binary log on the slave, but 
it's a large file, and is still going.

On the master, the binary-log-pos is 929084940. On the slave, it's way 
back at 889778259

Other than rebuilding the slave from a backup of the master, is there 
any way to get the replication backup up?

David