Have you tried repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair)?
It's also 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.
Also, don't use the Table of Contents tool found on the Ribbon's References tab. The way that tools inserts a Table of Contents imposes a severe performance hit on documents with lots of headings. Use TOC field you can insert via Insert|Quick Parts|Field instead.
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Cheers,
Paul Edstein
[Fmr MS MVP - Word]
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