There can be many causes, some bugs in your code (if there is native code) others bug in libraries you are using, others in the VM and/or OS. I suggest first checking out the trouble shooting guide:
http://java.sun.com/j2se/1.5/pdf/jdk50_ts_guide.pdf
To understand more about what you are seeing.Then try searching the forums and the bug parade for the specific problem. Posting in the JVM forum would be a better place
http://forum.java.sun.com/category.jspa?categoryID=41