Oracle Application Server Performance Monitoring and Tuning (CPU load high)

i have just hired by a company and my boss give me a performance issue to solve as soon as possible. I don't have any experience with the Java EE before at the server side.

Let me begin what i learned about the system and still couldn't find the solution:

We have an Oracle Application Server (10.1.) and Oracle Database server (9.2.), the software guys wrote a kind of big J2EE project (X project) using specifically JSF 1.2 with Ajax which is only used in this project. They actively use PL/SQL in their code.

So, we started the application server (Solaris machine), everything seems OK. users start using the app starting Monday from different locations (app 200 have user accounts,i just checked and see that the connection pool is set right, the session are active only 15 minutes).

After sometime (2 days) CPU utilization gets high,%60, at night it is still same nothing changed (the online user amount is nearly 1 or 2 at this time), even it starts using the CPU allocated for other applications on the same server because they freed If we don't restart the server, the utilization becomes %90 following 2 days, application is so slow that end users starts calling.

The main problem is software engineers say that code is clear, and the System and DBA managers say that we have the correct configuration,the other applications seems OK why this problem happens only for X application.

I start copying the DB to a test platform and upgrade it to the latest version, also did in same with the application server (Weblogic) if there is a bug or not. i only tested by myself only one user and weblogic admin panel i can track the threads and dump them. i noticed that there are some threads showing as a hogging. when i checked the manuals and control the trace i see that it directs me the line number where PL/SQL code is called from a .java file. The software eng. says that yes we have really complex PL/SQL codes but what's the relation with Application server? this is the problem of DB server, i guess they're right...

I know the question has many holes, i'd like to give more in detail but i appreciate the way you guide me.

Thanks in advance ...

Edit: The server both in CPU and Memory enough to run more complex applications