McDonald's Corp. is having trouble stomaching the competition. The world's biggest hamburger chain said Thursday that a key sales figure fell for the first time in nearly a decade in October, as it faced the double whammy of a challenging economy abroad and intensifying competition at home. The company, based in Oak Brook, Ill., says global revenue at restaurants open at least 13 months fell 1.8 percent for the month. The last time it dropped was in March 2003.