* TSX down 3.92 points, or 0.03 percent, at 11,854.21 * Energy stocks weighs on more weak China data * Potash Corp gains after U.S. slashes corn crop By Alastair Sharp TORONTO, Aug 10 (Reuters) - Canada's main stock index see-sawed o n F riday as more weak China data hurt some energy stocks, but fertilizer maker Potash Corp gained after the United States slashed the size of its corn crop. Adding to unease, Canada said its labor market unexpectedly shed jobs in July. A big undershoot in China's July exports soured the mood for the resource-heavy Toronto index, as concern about slower commodity demand offset hopes for stimulus measures. Weakness in the market is "primarily on the back of the Chinese data and at the same time confirmation that we're getting from international companies in the U.S. suggesting that global slowdown is certainly a concern for all of them," said Sid Mokhtari, director of institutional equity research at CIBC World Markets. While a majority of the 10 main sectors on the index were positive, the overall market was pulled negative by influential energy stocks including Canadian Natural Resources and Crescent Point. Fertilizer company Potash Corp bucked that trend, jumping 1 percent to C$ 43.16 after the US government slashed the size of its corn crop in the world's top grains exporter. Auto parts manufacturer Magna International also gained, adding 4.6 percent to C$43.73 after reporting a sharp jump in profit. Magna's strength in North America and Europe prompted Canaccord Genuity analyst David Tyerman to increase his stock target to $56 US from $53 US. By 10:30 a.m. (1430 GMT) the Toronto Stock Exchange's S&P/TSX composite index was down 3.92 points, or 0.03 percent, at 11,854.21. It had broken into positive territory earlier in the session. Domestically, Canada's economy unexpectedly lost 30,400 jobs in July in a third disappointing month for the labor market, suggesting the country's relatively hawkish central bank will stay on the sidelines for longer as growth fails to gain momentum. Research In Motion Ltd rose 5.2 percent after a Bloomberg report that International Business Machines Corp had expressed an interest in its enterprise-services business.