Java Database Connectivity (JDBC)

Every now and then, SQL queries and inserts from my servlet (on Apache) to an Oracle 9i database hang. Other connected tools, such as SQL*PLus, continue to run queries just fine. Re-starting Apache makes no difference. The only way to correct the problem seems to be to shutdown the database (with abort) and restart everything. Note that a regular shutdown from SQL*Plus also hangs.

My Apache web server (a development system) is shutdown & restarted periodically and my servlet does not explicitly close its JDBC connections at this time. I'm not sure if this is the cause of the problem or not.

Where should I be looking to diagnose these hangs?

Thanks very much.