Stocks struggled to hold onto modest gains in midday trading Wednesday. Bank of America (BAC) and Citigroup (C) are trading higher ahead of part two of the Fed's stress test. 31 U.S. banks passed round one of the central bank's examination of capital return plans. Shares of SanDisk (SNDK) are gaining after being named to Goldman Sachs' 'conviction buy' list. The firm likes the memory chip maker's expanding gross margins and attractive valuation. After the bell, investors will get treated to earnings results from Shake Shack (SHAK) and Krispy Kreme Doughnuts (KKD). Shake Shack is dropping following Tuesday's big gains.