Gains in Amazon > and Costco Wholesale Corp COST.Ohelped limit losses in the Nasdaq. By Noel Randewich Sept 26 (Reuters) - Wall Street slid deeper into a bear market on Monday, with the S&P 500 and Dow closing lower as investors fretted that the Federal Reserve's aggressive campaign against inflation could throw the U.S. economy into a sharp downturn. Confidence among stock traders was also shaken by dramatic moves in the global foreign exchange market as sterling GBP=D3 hit an all-time low on worries that the new British government's fiscal plan released Friday threatened to stretch the country's finances.