SBS 2011 Activesync Problem

Have an SBS 2011 STD site  - a few users are experiencing ActiveSync issues as follows:

For these accounts email will update but it lags a bit and every few syncs the user will get "Connection to Server Failed".  Mail does eventually sync.  Contacts sync OK.  The calendar however simply refuses to sync at all.  The main user in question has a packed calendar with several appointments per day.  Things tried so far:

Recreating mailbox  - when mailbox was empty - test email, calendar appointments and contacts all synced OK.  As soon as data was reimported however, the above symptoms reappeared.
Moving mailbox to another DB - the thought here was that any corrupted items affecting the sync would not be moved as I picked the option to not move corrupt items - move completed with 0 corrupt items - problems remained.
Recreated ActiveSync virtual directory - no affect
The next thing that I will try is to separate all calendar items older than 7/1/13 to a PST file in case there is still something in the calendar that is preventing the sync from occurring.

Mobile devices are fully updated iphone 5 and ipad.  Exchange is SP 1 with all the rollups.  I want to install SP3 but am not comfortable doing that until the weekend so I have a few days of buffer in case the SP breaks Exchange.

Just as a clarification - other users with Apple devices are syncing fine.  Also - nothing getting logged in App Event Log related to ActiveSync errors.

Any other advice would be appreciated. Thanks.