Stocks gave up more ground in afternoon trading Thursday as a slew of leaders were under pressure. The Nasdaq slumped 1.5%, while the Dow Jones Industrial Average and S&P 500 lost 1% each. Turnover continued to track mildly higher on both major exchanges in the wake of the Supreme Court's decision to uphold the ObamaCare health legislation. Ross Stores (ROST), which has been a slow and steady gainer in recent years, dropped 4% in heavy trading.