Please check : $LOG_HOME/ora/10.1.3/j2ee/oacore/oacore_default_group_1/application.log
There might be possibility of Java Heap Size issue.
You can follow the steps in this note to gather the logs
R12, 12.1 - How To Enable and Collect Debug for HTTP, OC4J and OPMN (Doc ID 422419.1)
It is important to gather all the application tier logs to build a clear picture of what is going wrong.
If you provide the logs I will have a look for you