I told myself that if we could get 2 full months without a recurrence, I'd consider the problem solved. Alas, this evening, after about a month and a half, it reared its ugly head again.
Ran Live Update at 8:06 PM EDT with the following results:
Norton 2012 Web Protection Definitions: Failed
Norton 2012 Smart Virus Definitions X64: Success
Norton Pulse Updates: Success
Norton 2012 Anti Spam Definitions: Success
Norton Activity Map Data: Failed
Ran LU again at 8:09 PM EDT, with the following results:
Norton 2012 Web Protection Definitions (hub); Success
Norton 2012 Web Protection Definitions: Failed
Norton Activity Map Data: Failed
Ran LU a third time at 8:23 PM EDT with the following results:
Norton 2012 Wep Protection Definitions: Success
Norton Pulse Updates: Success
Norton Activity Map Data: Success
Can't say for sure that this is the same problem that has been going on for several months, but it behaves the same. Had not (to my recollection) previously encountered the Web Protection Definitions (hub) entry. But haven't been checking the update details unless there is a failure, so haven't looked at the details in the last month or so.