Java out of memory problem

We are running Linux RedHat7.1 and resin 2.0.3 on our production servers with jdk 1.3
We constantly get java.lang.outofMemory errors. The number of java threads is set to 30 right now. I upped the memory allocated for java threads to 512 from 256 and it helped a little two and a half weeks we haven't recieved on out of memory alert. However, today I got it again.
Doe anyone know what to do and how to find the memory leak?
Thanks!

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