I take it you see no errors or exceptions in the JBoss logs? Does the logfile show a shutdown or an abrupt stop? If it stopped abruptly, it would help if the JVM left a crash message file (perhaps in jboss/bin?? Not sure where this might be if JBoss is run as a service).
Also check that the startup command does not use the "-server" option. Because of a JVM bug this causes problems for some, and your situation would be typical under light or no load.
Thanks your your input. How to register Jboss as service in Window 2000?