A vertical line that ranges from a thin black line to a thick red line periodically appears then eventually disappears then eventually reappears on the external Dell 2408 WFP display attached by mini displayport-to-DVI from my maxed out 2011 Mac Air 13". There is no apparent reason. I've changed the resolution with no effect. I regularly clean house by booting to option-command-p-r and by running sudo periodic, verify permissions and verify hdd. I have not seen this in the Bootcamp Windows 7 partition. Any thoughts?