Merrill Cuts Citigroup Rating, Stock Down  NEW YORK (Reuters) - Citigroup Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=C.N target=/stocks/quickinfo/fullquote"&gt;C.N&lt;/A&gt; shares fell more  than 2 percent early Monday, after Merrill Lynch analyst Guy  Moszkowski cut his stock rating to "neutral" from "buy," citing  recent ethics problems.