Random crashes on getConnection

I get random exceptions on GetConnection() on a Quartz-based job.
Sometimes everything works, and sometimes I get a "Protocol Error.
Session setup failed". Does anybody know what might be happening?

Client: Both postgresql-8.3-604.jdbc4.jar and
postgresql-8.4dev-700.jdbc4.jar.
Server: Postgres 8.1. The databases are stored in LATIN9 encoding.
JVM: java-6-sun-1.6.0.12

SEVERE: com(dot)androme(dot)phemium(dot)statistics(dot)EligibleCustomerList(at)3b1d04
org.postgresql.util.PSQLException: Protocol error. Session setup failed.
at
org.postgresql.core.v3.ConnectionFactoryImpl.readStartupMessages(ConnectionFactoryImpl.java:481)
at
org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:101)
at
org.postgresql.core.ConnectionFactory.openConnection(ConnectionFactory.java:66)
at
org.postgresql.jdbc2.AbstractJdbc2Connection.<init>(AbstractJdbc2Connection.java:124)
at
org.postgresql.jdbc3.AbstractJdbc3Connection.<init>(AbstractJdbc3Connection.java:30)
at
org.postgresql.jdbc3.Jdbc3Connection.<init>(Jdbc3Connection.java:24)
at org.postgresql.Driver.makeConnection(Driver.java:393)
at org.postgresql.Driver.connect(Driver.java:267)
at java.sql.DriverManager.getConnection(DriverManager.java:582)
at java.sql.DriverManager.getConnection(DriverManager.java:207)
at
com.androme.phemium.statistics.EligibleCustomerList.get(EligibleCustomerList.java:34)
at
com.androme.phemium.statistics.StatisticsCollectionDispatcher.execute(StatisticsCollectionDispatcher.java:79)
at org.quartz.core.JobRunShell.run(JobRunShell.java:202)

PS: Sorry if this message duplicates.
--
<b>Ramon Poca</b> <br/>
<a href="mailto:ramon(dot)poca(at)androme(dot)es">ramon(dot)poca(at)androme(dot)es</a>