Rebuild your Active Document

Your document is corrupt, or it is so heavily edited that it is behaving badly.
Use the little macro "RebuildActiveDocument" to grab all your text, unformatted, and rebuild the document.
CAUTION: make a copy of your document using Windows Explorer before running this macro.

Sub RebuildActiveDocument()

    ' Get the name of the source ActiveDocumentument

    Dim strFilename As String

    strFilename = ActiveDocument.FullName

    ' Get the stoty-content of the ActiveDocumentument

    Dim strStoryContent As String

    Dim lng As Long

    For lng = 1 To ActiveDocument.StoryRanges.Count

        strStoryContent = strStoryContent & ActiveDocument.StoryRanges(lng).Text

    Next lng

    ' Close the ActiveDocumentument WITHOUT saving changes

    ActiveDocument.Close (wdDoNotSaveChanges)

    ' Create a new ActiveDocumentument

    Documents.Add

    ' Insert the text file

    Selection.TypeText (strStoryContent)

    ' Save the ActiveDocumentument with the original name

    ActiveDocument.SaveAs (strFilename)

End Sub