Your description isn't something that I've ever seen with a normal mailmerge. Perhaps your mailmerge main document is corrupt, or there's a fault in your Office installation.

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.

If that doesn't resolve the issue, try repairing the Office installation (via Programs & Features > Microsoft Office > Change in the Windows Control Panel)

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]