Problem with background thread in Tomcat 5.5.9

I am trying to run a servlet in Tomcat 5.5.9 which has a background thread.
This thread uses JDBC, and in particular the 8.1 Postgresql JDBC driver.  The
foreground bits of the servlet work quite properly.

But when the executeQuery function goes off and tries to get a
PhantomReference object it fails with a ThreadDeath exception.

       at
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1221)
        at
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1181)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
        at
org.postgresql.core.v3.QueryExecutorImpl.registerParsedQuery(QueryExecutorImpl.java:1092)
        at
org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1177)
        at
org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:188)
        at
org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:430)
        at
org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:346)
        at
org.postgresql.jdbc2.AbstractJdbc2Statement.executeQuery(AbstractJdbc2Statement.java:250)

Anyone got any ideas why this might be happening and what I can do to fix it?

David