There is a figure in connection Tab that shows the rate of memory usage and cpu and some other usefull reports.
By the way , with this condition that there is no note from weblogic  and i have no overall vision on problem.
I can only give some global comments:
1- since u use hibernate  , there may be some problem  with hibernate session factory bcuz it is so weak and can not afford under high load.
2- ur pooling method for connections in weblogic is not well .
3- changing weblogic JVM to JRockIt may be a nice try.
4-there may be problem in oracle  with oracle query caching.here i put a question.how is repsonse speed of ur appliation?
5-I dont know about carcked version of weblogic with 5 users  license;the version that we use has no limit for users. may be this is the reason of ur prob.-i have a web  application with the same spec but with nearly 3 times more users and it works.-