You get the ORA-1555 message when you try to access data that someone else is modifying, and Oracle has lost the information that it needs to construct a read-consistent image of that data to present to you. Since you are using Oracle9i's new UNDO tablespace, the ORA-1555 error is an indication that your UNDO_RETENTION parameter is not large enough. You can monitor this by querying the SSOLDERRCNT (Snap Shot Too Old Error Count) column of V$UNDOSTAT. Increase the UNDO_RETENTION parameter and these errors will go away.
I hope it helps you.