McDonald’s Corp (MCD.NYSE) reported weaker-than-expected quarterly sales at established restaurants on Thursday as fewer diners visited the fast-food chain. The company also warned that sales would fall short of analysts’ expectations again in January. The world’s biggest restaurant chain by revenue has reported subpar sales for five straight quarters. The causes include self-inflicted operational stumbles, weak demand and intensified competition from resurgent rivals such as Wendy’s Co (WEN) and Burger King Worldwide Inc (BKW). Chief Executive Don Thompson’s efforts to lift earnings in the 18 months since he took the company’s helm — by tweaking menus and shaking up management — have not borne fruit. The pressure also is on him to boost McDonald’s share price. The stock is up just 7 percent since Thompson became chief executive on July 1, 2012, well behind the 27-percent jump in the Dow Jones Industrial Average index, which includes McDonald’s as a component.