rman instance down suddenly without reason

[chuikingman]

Hi,
I make rman warmbackup in cromjob .
but it is failure .
Recovery Manager: Release 9.2.0.7.0 - 64bit Production

Copyright (c) 1995, 2002, Oracle Corporation. All rights reserved.

connected to target database: RCA (DBID=2453491915)
RMAN-00571: ================================================== =========
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ================================================== =========
RMAN-00554: initialization of internal recovery manager package failed
RMAN-04004: error from recovery catalog database: ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
SVR4 Error: 2: No such file or directory
07/24/09 02:13:51: Tar Backup File to /dev/rmt/0

I find the rman instance is down and I startup rman instance again.
Then the rman warmbackup is ok again .
I check the alert.log but there is no data about why the rman instance down .
Any idea how to check/know why the rman instance down suddenly ??
Any trace or method to collect information to prevent rman instance down suddenly ???
please advice and suggest .