Stocks gripped solid gains through most of Thursday’s session, but then gave back most of the advance in the last hour. The Nasdaq posted a 0.4% pop after being up 1.1%. The S&P 500 turned a 1% gain into 0.2%. The IBD 50 also rose 0.2%. Volume fell sharply on both major exchanges. The negative reversal was another mark against this uptrend, and it wasn’t the only bad sign.