Hypersonic SQL unstable

We are developing a Java application as a front end to a HSQL DB. It mostly works fine but seems to be unstable and will sometimes give very unexpected errors like unique key violations and integrity constraint violations at unexpected times. We know what can cause these types of errors but they do not have a logical reason to occur where they occur in our application and seem to occur on some PCs but not others. We are using JVM 1.3 but Visual Cafe v4.0. Does anyone have any experience with instability in HSQL and how to get around it. It is causing us a serious headache and is consuming a lot of very valuable development time.