Data File not closed down properly

A month or so ago I posed a question about soon after upgrading to Windows 7,
I experienced an error every time I open Outlook saying that the data file
was not closed down properly, and it fixes itself over about 10 minutes. It
was suggested that add-ins could be the problem, or my using an iPhone
Outlook synchronisation app.

I have since then disabled all the Add-Ins I can (except Microsoft Office
Groove Proxy for Outlook, Microsoft Outlook Mobile Service, Microsoft VBA for
Outlook Add-In, OneNote Notes about Outlook Items, Outlook Change Notifier,
Send to Bluetooth and Windows Search Email Indexer which do not let me do
so). stopped the iPhone app and Nelson Email Organiser (whithout which I will
be lost) and the problem persists.

The problem is inconsistent - sometimes it happens, and sometimes not.
Mostly it occurs

By the way - I have a very big .pst file (1.4 GB or so) could this be a
problem? I have archived a great deal, but that did not seem to make a
difference.

Any other thoughts?