Outlook 2007 - large OSTs, sync problems

In our environment we have a client who manages multiple calendars, that is, she is a delegate for 9 different execs and manages 9 calendars in Outlook.  Exchange server is v2010, Outlook is 2007 SP2 on all workstations.

We are finding that the user has periodic problems with sync of changes she makes to the execs calendars; from time to time she will schedule a meeting or change a meeting on one of their calendars and while the invites and updates go out to the attendees, and the copy of the exec's calendar in her local OST folders is updated, the meeting is not synced to the exec's Calendar folder on the server.

I noticed that the user's OST is larger than 5GB, which I found unusual, I rarely see OSTs in that range.  I also noticed that in the user's Windows Event Log, there are multiple instances of an event that says that the OST is being reconciled with the indexer because the store was last opened on a different machine - an impossibility, since the OST is on a local hard disk and not shared with any other computer.

Looking for suggestions on resolving this - my theory is that the OST is so large that Outlook can't effectively and efficiently sync it against the Exchange servers, or possibly that the OST is corrupt.