Hi, and welcome to FineTunedMac.
> Any idea why my monitor would start showing green lines?
Please let us know your results, and please respond to grelber's request.
Actually, before trying those tests I turned the monitor (but not the computer) off overnight. The lines were gone in the morning.
I suspect a thermal problem of some kind.
Yesterday was a hot day and the room with the computer was unusually hot. Today is a cooler day and so is the computer.
I also suspect it was the card overheating rather than the display because the screen grabs I took had the lines and screen grabs seem to grab the video signal rather than monitor.
I'm going to write this off as a "buy an air conditioner" solution.