Thank you for the suggestion. I've tried this; the first thing I realised is that you have to reset things like margins, page size and indents ( I don't know whether or not it's supposed to, but 'keep source formatting' doesn't seem to affect that ); given that it's quite a large document, I'm not sure whether or not it will prove viable to manually reset all these things throughout the entire course of the document, but in the meantime, I'll keep an eye on it and see whether the original problem re-occurs or not.
At any rate, thank you for the suggestion; I gather from what you say that the most likely cause in the first place was a corrupted document?