It sounds like you are facing a challenging intermittent issue with your Java application querying the DB2 database. Here are some potential areas to investigate that might help resolve the problem:

**Resource Limits**: Look for resource limits on your DB2 instance, such as connection limits or memory usage, that may affect query execution.

**SQL Execution Context**: Verify if there are any environmental differences between running your query through the application versus the DB2 client. For example, check user permissions or roles associated with the connection used by the Java application.

**Debug Logging**: Add debug logging in your application to capture query execution, parameters, and connection details to isolate when the behavior changes.

**DB2 Configuration**: Review DB2 configuration settings, such as optimization or locking options, that might impact query behavior intermittently.

If the issue persists after investigating these areas, consider enabling detailed DB2 tracing to gather more information about query execution and performance. This may provide additional insights to help pinpoint the underlying cause.