Illustrator :: Shade Of Objects Changes Depending On Their Vertical Position On Artboard

This is  a recent phenomonom, the shade of objects changes depending on their vertical position on the artboard. If I move an object from the top of the artboard to the bottom it becomes lighter, and these changes remain even after it is saved as a png- this is driving me crazy!!! it is very slight, but still enough to cause a headache. If you look at this image you can see that the top blue box is slightly darker than the blue box at the bottom, event though this is the exact same box copied and pasted down the artboard.