Stocks Off on Exxon Downgrade, Oil Price  NEW YORK (Reuters) - U.S. stocks slipped on Thursday after  Exxon Mobil Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=XOM.N target=/stocks/quickinfo/fullquote"&gt;XOM.N&lt;/A&gt; was downgraded by a brokerage and oil  prices rose, raising investor concerns about the health of  corporate profits and economic growth.