Hello Toby,
I would request you to please try Migrating to the latest Symantec Endpoint Protection 11.0.6300 (RU6 MP3) and let us know if the same remains.
Migrating to Symantec Endpoint Protection 11.0.6300 (RU6 MP3)
http://www.symantec.com/business/support/index?page=content&id=TECH155655
Incase, if the issue remains, I would request you to open a web case and contact Technical Support.
QuickStart Guide - Create and Manage Support Cases in SymWISE
http://www.symantec.com/docs/HOWTO31132
How to update a support case and upload diagnostic files with MySupport
http://www.symantec.com/docs/TECH71023
To guide you better, the Symantec Technician would require certain logs (SST Logs and system DUMP logs) to understand the root cause of the issue in your environment.