Hi Ruth,
Given that the problem is intermittent and not repeatable, it may be very difficult to isolate what's causing it. A bad network connection might cause this for example.
If you insist in retrieving all your transactions at the end of the month, then I'd just recommend that you be religous in your backups. Disk space is cheap. Perhaps at some point you'll be able to identify a sequence of steps that you take to re-create the problem.
I know this isn't very helpful, but until the problem is completely repeatable with a set of data, there's very little we can do.
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Andrew Bates