U.S. economic activity cools in August-Chicago Fed Thursday September 25, 10:05 am ET NEW YORK, Sept 25 (Reuters) - A gauge of national economic performance slipped in August to reflect below-trend growth, mostly due to the worsening labor market and a pull-back in industrial output, the Federal Reserve Bank (News - Websites) of Chicago said on Thursday. The Chicago Fed's National Activity Index fell to -0.28 from an upwardly revised 0.05 in July. Any negative reading suggests economic growth running below trend, which most economists peg around 3.5 percent for gross domestic product. The slip in the index comes even...