This happens to me all the time using the healing brush or cloning tool. I allready turned gpu accelleration off but it still exists! Also updated graphics card drivers. No go.
It exists on both machines, using different graphics cards and different cpu's.
I am using windows 8, windows 7 and photoshop cs6 extended.
It doesn't seem to be a gpu problem since i turned that off.
Hope someone can point me to a solution.