Is this problem occurring only with JBoss WebServer or on JBoss AS, too?
If it's happening on JBoss AS (Application Server) you probably want to post your question instead on the "Persistence,JBoss/CMP, Hibernate, Database" forum:
Neither that forum or this one seems to get much attention from those in the know, however.
You might want to add some timing code around your JDBC calls to see how long they are taking to return. If the time to complete the query exceeds the timeout configured for the datasource you could see errors like this. Check out the wiki for how to set timeouts: