Xalan errors fixed by reboot

We are currently using Xalan to produce PDF docs usinf xsl:fo. This has been deployed to our production environment without any issues in testing. However, the production environment is intermittently getting into a state such that it throws Xalan errors of

javax.xml.transform.TransformerException: org.apache.xml.utils.WrappedRuntimeException: The output format must have a ‘{http://xml.apache.org/xalan}content-handler’ property!

We hae found that rebooting the server will eventually fix this problem and any failed messages cn be successfully resubmitted with trading networks.

I have found some articles that suggest this error can be caused by using Xalan with version 1.4 of the JRE. We are using IS 6.1 with the supplied JRE (1.3.1) but JRE 1.4.2 is also installed on the machine.

Any ideas appreciated!