The most likely causes of such behaviour are document corruption and/or faulty drivers for printers, display and/or mouse. If it affects only a small number of documents, document corruption is the most likely cause; other wise a faulty driver is likely.

As previously advised, 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, closing the old document and saving the new one over it.

In case it's a faulty driver, check the various makers' websites for updated drivers. Faulty font files can also be an issue, but they're much harder to isolate.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]