Found this technote
BUG REPORT: In multiple versions of NetBackup the nbjm process deadlocks causing backups to stop processing.
http://seer.entsupport.symantec.com/docs/317755.htm
Check your system for core dumps, and clear them out.
This states the following
This issue was scheduled to be addressed in the following release:
- NetBackup 6.0 Maintenance Pack 8 (MP8)
Not seen 6.0 MP8