Do you have any third party backup software backing up the database when that happens?

According to Metalink, such errors (exactly the same error stack) are caused by: "....Typically this error is caused by 3rd party backup software placing a lock on an Oracle file and then when Oracle tries to write to it, the OS 33 is generated and the database will crash."

Check metalink note 130871.1 and doc.id 352819.999 in Metalink's Technical Forum.