I'm getting horizontal scan lines in some of my windows on an older MBP. They appear on the display and external monitor, and appear to be occurring when rendering, as I can drag a window they've appeared on between the built in display and external monitor and the lines travel with the window between monitors. They are not clean lines, but rather formed out of digital noise. I already had the motherboard replaced for various other issues, and these scan lines were cropping up then but disappeared for a while after the replacement. Am I screwed? Whats the likely culprit? I had them on an older machine where the display wires had been slightly severed, but I don't think there's been any damage to any of the wiring this time. Any suggestions are much appreciated!