One of the fields being merged may have a size which pushes subsequent pages
down one line forcing a new or blank page.  You would have to create some
controls of the merged fields to see which one may be doing this but I do
not know of any scenario which would natively cause this behavior.  Tyr and
decrease th margin size of the document to accommodate an extra line or two
and see if that gets it!

I never liked Mail Merge as an automation (too many untrollable factors), we
have created solutions which do search and replaces that way people don't
have to know special formats to make the mail merge work.  You simply add
something like <+firstname+> as a field and replace the data from an xml
stream.  Give the users (librairans, etc) a list of fields they can use and
they can create as many documents as they like.  Furthermore you can use
this technique to create Master documents and add combinations of documents
on the fly for a more robust printing solution (tougher to do in an
automated mail merge).

Good Luck!
john