My Safari 4's been doing the same thing ever since Digg released the Digg Bar. At first I thought this was a hardware problem, as people had similar issues when their video card was heating up, but I found every single time I clicked on the url link on the digg bar (and thus removing the digg bar) the lines would go away and never come back until I hit another Digg link. Not sure whether it's Digg or Safari, but a software issue of some sort...