Failing to maintain its initial gains, the Sensex at the Bombay Stock Exchange had plunged 429 points, or 2.3 percent, to 18,457 in afternoon trade, a short while ago, on fresh selling by investors, amid renewed rupee weakness. Earlier in the morning, however, the Sensex had gained 121 points, or 0.6 per cent, to 19,007, amid firm regional bourses. The Sensex had rallied almost 920 points in the previous four trading sessions.Stock markets in Japan, China, Hong Kong, Singapore, South Korea, Indonesia, and Taiwan had added between 0.1 percent and 3 percent, today, on data indicating a pickup in global manufacturing.