Wall Street was bouncing back Tuesday from Monday's rout that knocked 512 points off the Dow Jones industrial average. In a day of wide swings, the Dow average first shot up 143 points, then tumbled by 139, only to begin rising again. With two hours of trading left, the Dow stood at 7,699.40, up 160.40. Broader market indicators also were higher, including the technology-heavy Nasdaq. President Clinton added his voice to efforts to restore calm to the shaken markets. "We believe our fundamental economic policy is sound," he said Tuesday in Russia, echoing comments Monday by Treasury Secretary Robert Rubin.