Insurance Probe Woes Deflate Stock Rally  NEW YORK (Reuters) - U.S. stocks were flat on Tuesday after  a strong opening as declines in health insurers offset  better-than-expected earnings from tech bellwethers IBM Corp.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=IBM.N target=/stocks/quickinfo/fullquote"&gt;IBM.N&lt;/A&gt; and Texas Instruments Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=TXN.N target=/stocks/quickinfo/fullquote"&gt;TXN.N&lt;/A&gt;.