Overclocking is by nature very hit and miss or trial and error. If you get artifacts when changing voltages you are setting the voltages out of the range in which the graphics card will work properly. Each card is different, even with the same model you may get one result but not be able to duplicate it on another identical card. This is the fun in overclocking, trying to optimize the settings for your specific card and squeeze the most out of it, sometimes you can squeeze more other times not so much.