It's possible the document has acquired some of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

It's also possible there's a fault with Office and/or your Endnote Addin. Repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair) will usually fix the former; you will have to follow whatever 'repair' process Endnote provides (perhaps a reinstall).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thank you for the reply.

The symptoms seem to fit a repagination issue. It goes away in draft view.

I've decided to give up for now and just put up with it, but I'll have a go at fixing it once the work is done - hate being beaten by things like this!