Isolation issue with Oracle in EJB

We are experiencing problems (ORA-08177 can not serialize exception during long transaction) during concurrent access from multiple transaction. We have all our Session and Entity Beans with
TX_ATTRIBUTE : TX_REQUIRED
ISOLATION LEVEL : TX_SERIALIZABLE
Persistance : BMP
Transaction : CMT
Is there KNOWN problem with Oracle databse-- basically the problem being that Oracle's isolation levels don't map well to the EJB specification.
I tried IBM web site for tech Support, this kind of bug not reported any where.
Could some one give some insight?
Thanks
Alagan