ORA-01034: Oracle not available
ORA-27101: shared memory realm does not exist.
Make sure no other related services are trying to start before you get the main service up and running. It sounds like some other service that needs the database to be up and running is trying to start and can't find the database. It's likely a timing thing. Since it doesn't happen all the time, that would be my guess. Check all the related services and make sure nothing is trying to start out of order.
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