NEW YORK (Reuters) - Stocks ended slightly higher in a third session marked by late-day swings, but the Nasdaq fell after NetApp gave a weak revenue forecast, casting doubt on the outlook for tech spending. Major indexes were lower for much of the session, as investors found little reason to buy following three days of gains. In addition, economic figures suggested slowing demand in both Europe and the United States. However, Wall Street reversed course late in the session and the S&P extended its gains to a fourth straight day. Underscoring the vulnerability of U.S. ...