Another wave of volatile stocks roiled Wall Street on Wednesday, prompting some exchanges to halt trading and reinforcing anxieties that another software problem would turn off investors. U.S. exchanges said they were looking into potentially erroneous trading in more than 100 securities that saw notable upswings in price. The swings occurred amid buy and sell orders on electronic trading platforms, causing Knight Capital Group—one of the largest trading firms—to suddenly lose a fifth of its market value. Radio Shack Corp. also experienced price swings. Meanwhile, investors called on the Federal Reserve to help stimulate the market.