I don't know of any obvious reason why this would happen except a formatting
error of some kind in the document.

Unless you are doing something such as using an unusual font, putting text
into a narrow table or column, the chances are that there is some corruption
in the document you are using to create your Mail Merge Main documents. If
you are always starting with a new document, then there could be a problem
with your "default template", normal.dot.

It's probably worth going through the steps at

http://word.mvps.org/FAQs/AppErrors/ProbsOpeningWord.htm

even though "opening Word" isn't the problem you actually have.

Peter Jamieson