By John S. McClenahen Japan's economy, second largest in the world after the U.S.'s, is showing some signs of weakness again. The New York-based Conference Board says its leading economic index for Japan fell for the third consecutive month in October, the latest month for which data are available. Japan's leading index now stands at 98.6 (1990=100). "The leading index has declined at a 3.6% rate over the past three months, but this follows a very rapid 10.3% growth over the previous three months, which is why the six-month growth rate is still 5.2%," says the Conference Board, a business research group. "The recent behavior of the leading index suggests that the [Japanese] economy will continue to grow, probably above the 0.7% average of the last two quarters but below the 5.2% average of the preceding year."