Stocks posted modest gains in mixed volume Thursday, as the S&P 500 struck an all-time high. The Nasdaq edged up 0.1%, while the S&P 500 added 0.3%. The IBD 50 fell 0.5%. Volume rose slightly on the NYSE but eased on the Nasdaq. Action has been sleepy on the Nasdaq recently. On Thursday, volume on the Nasdaq fell for a fourth consecutive session. The index also has kept to a tight price range.