ORA-03113: end-of-file on communication channel

Environment: 9.2.0.7, Linux x86.
Error : ORA-03113: end-of-file on communication channel.

This happens in a script(unattended mode) which calls shutdown and startup, it does not happen for every run. Once in a week or once in two weeks. No change to the script and environment. We do manual shutdown or start up, when this issue happens. That works. Im not able to understand why its failing for run in 100s of runs and whats causing. Last night it happed after instance is dismounted during shutodwn immediate.

I did set event 3113 to see if that gives me a clue. No luck, no traces are created.

Does anyone have any idea on this ?

Thanks!!!

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