Stocks stumbled at the opening bell Thursday but quickly reversed en route to an up day. The Nasdaq added 0.6%, while the S&P 500 lagged with a 0.2% gain. The small-cap S&P 600 popped 1.2%, smoothing over the previous session’s 1% dent. The IBD 50 tacked on 0.3%. Volume ran higher on the Nasdaq but lower on the NYSE.