Help analyzing glassfish server hang problem

Question

We are running a glassfish server with around 20 jax-ws metro web services. The server specs are Core2Duo with 8GB RAM. We are using a single http listener for all the web services. Development is set to True. Request Thread Count is 2 and Acceptor Count is 1. The Minimum and Maximum Heap Sizes are 1GB and the Perm Gen is set to 512MB. The services access an Oracle database via a Hibernate layer and there are many interservice calls between the services. The front end is ASP.Net. Our problem is that when 4-5 users try to access the application simultaneously for some time (1 hour) the glassfish server hangs with the CPU going to 100% but the memory utilization is around 10-11%. We are not able to find any pointers as to how to debug this problem. On some instances the log file gives java.lang.OutofMemory Exception : PermGenSpace. But this is also not everytime, i.e. on many occassions the log file does not give any error on hanging. Also the glass fish server does not start if we try to increase the Perm Gen Space. We need some direction on how to diagnose and move towards the solution to this problem. The Glass Fish Version we are using is v2.1. We have the following observations: 1. Adding more http listeners (1 listener per 4-5 services) does prolong the failing time but not with much effect. 2. When calling some of the heavy services (one by one operation) with SOAP-UI we also get the hang problem when running many threads simultaneously. (e.g. 8-10 threads) 3. We have observed that when calling with SOAP-UI a service operation (which does not call any other services) rarely hangs while a service calling other services hangs much frequently.