Stocks gave up gains and turned mixed around midday Thursday, after House Speaker John Boehner sounded a negative note about talks to avert the fiscal cliff. The Dow Jones industrial average lost 0.1%, while the S&P 500 was flat. The Nasdaq added 0.2%. Volume was up from Wednesday's level for the Nasdaq, but down for the NYSE. The indexes had all been positive earlier Thursday, but then Boehner said in a news conference that Washington had made no