NEW YORK (MarketWatch) — After switching between gains and losses, U.S. stocks ended Wednesday’s choppy session essentially flat. Investors remained jittery, with uncertainty hanging over Greece’s future weighing on markets. The S&P 500 SPX, +0.00% ended unchanged at 2,068.53. The Dow Jones Industrial Average DJIA, -0.04% closed 6.62 points lower at 17,862.14. The Nasdaq Composite COMP, +0.28% outperformed other indexes, ending 13.54 points, or 0.3%, higher at 4,801, largely thanks to a 2.3% gain in Apple Inc, it’s biggest component.