GM's U.S. Sales Surge  DETROIT (Reuters) - General Motors Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=GM.N target=/stocks/quickinfo/fullquote"&gt;GM.N&lt;/A&gt; on Friday  said its U.S. sales soared a surprising 20.5 percent in  September, but the U.S. automaker also cut production and  raised incentives as it grapples with high inventories of  unsold vehicles.