Hibernate parameterized sql query slow and active oracle sessions
I don't know if this is your issue, but Oracle peeks at bind variable values when parsing a query and then saves the query plan for future executions so it doesn't have to keep parsing the query each time it is run with a new set of bind variables. But every once and a while the query is re-parsed. If some unusual bind variable values happen to be passed during a parse then a bad plan is stored and used. It is kind of the curse of bind variables. They reduce parsing but can flip the plan around on atypical bind variable values when queries are parsed again. Hints can help. We use SQL Profiles to lock in plans of queries with bind variables that tend to change plans. Sometimes you can customize when and how optimizer statistics are gathered so that a good plan is created regardless of what values are passed into the bind variables.
Anyway, its something I see all the time and may or may not be your issue.
Bobby