Java SE (Java Platform, Standard Edition)

I develop a web application which runs on top of apache tomcat in JVM 1.6.0_20 on a RHEL 5.5. The server is a 64Bit dual quad core with 12 GB RAM and initially we had -Xmx5500m but I also tried it with just 1.5GB and in both cases under heavy load it crashed with a segfault. The only output I get is in /var/log/messages:

Jul 26 11:21:32 server kernel: java[5149]: segfault at 000000005566ffe8 rip 00002aab1b68059d rsp 000000005566ffd0 error 6

Does anybody have any hint what crashes the JVM? As far as I can see from the application stats the memory given by -Xmx is by far enough.

Thanks,
Stefan