It seems to be an oracle bug 6499872. There is no patch for 10.2.0.2., so I ended up recreating the undo tablespace.

The syntax for the undo tablespace creation was ...
CREATE UNDO TABLESPACE PSAPUNDO DATAFILE
'/oracle/XXX/YYYY/undo_23/undo.data23' SIZE 8190M AUTOEXTEND OFF,
.....
.....
......
ONLINE
RETENTION NOGUARANTEE
BLOCKSIZE 8K
FLASHBACK ON;

Thanks
Jay