Merrill Cuts Citigroup, Shares Drop  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 as  much as 2.75 percent on Monday after a prominent Wall Street  analyst downgraded the world's largest financial services  company, citing a string of recent ethics problems.  Analysts say the blows to Citigroup's reputation might be a  function of the New York-based giant's size and far-flung  businesses.