Java Database Connectivity (JDBC)

We have our application deployed in Development and Testing environment. While we never get a Staleconnection exception in Development, it happens intermitenly on Testing environment on a statement that invokes prepareCall/prepareStatement. We are using connection pool and the pool size is 10. The connection orphan time is was increased to 3000, but it didn't help

Both UAT and dev have the followign configuration

Websphere version 3.5
Application on a Portal Server called Epicentric.
Sybase Jconnect Version 5.2

Why could this be happening? We have taken care that before closing the connection in finally blocks (connection.isClosed()) is checked.
The stack trace is

com.ibm.ejs.cm.portability.StaleConnectionException: JZ0C0: Connection
is
already closed.
at java.lang.Throwable.fillInStackTrace(Native Method)
at java.lang.Throwable.fillInStackTrace(Compiled Code)
at java.lang.Throwable.<init>(Compiled Code)
at java.lang.Exception.<init>(Compiled Code)
at java.sql.SQLException.<init>(Compiled Code)
at com.ibm.ejs.cm.portability.PortableSQLException.<init>(PortableSQLException.java:22)
at com.ibm.ejs.cm.portability.StaleConnectionException.<init>(StaleConnectionException.java:19)
at java.lang.reflect.Constructor.newInstance(Native Method)
at com.ibm.ejs.cm.portability.PortabilityLayerImpl.translateException(PortabilityLayerImpl.java:151)
at com.ibm.ejs.cm.pool.ConnectO.translateException(ConnectO.java:1000)
at com.ibm.ejs.cm.proxy.ConnectionProxy.translateException(Compiled Code)
at com.ibm.ejs.cm.proxy.ConnectionProxy.prepareCall(Compiled
Code)