A late round of selling spoiled what was looking to be a strong comeback for the stock market Tuesday, highlighting the fragile state of affairs. The S&P 500 slumped 1.4%, while the Nasdaq lost a mild 0.4%. Gains in Internet, biotech and software stocks cushioned the tech-heavy index’s fall. Both indexes ended at their intraday lows and extended their losing streaks to six sessions. The S&P 500 ended a hair above Monday’s low.