Can anyone explain why the 12-18-2013 build is forcing single memory allocation when the 12-10-2013 version didn't? And how can I turn it off?

On a GTX 660m: In the 12-10-2013 build I was getting 50khash/s, now in the 12-18-2013 build the program autotunes and says it will gets 65 khash/s then sharply decreases to about 20khash/s after about a minute.