hi,the problem you having is not software problem. unfortunately, the lcd panel backlight on your computer have pixel and some of those pixel are defective. most pixel on lcd panel work together in lines
that you if one pixel go back other pixel within that line will likely to go bad with it
the only way to fix it is to bring to apple store and have them replace the lcd panel
imac 2006 - 2010 is common for bad pixel lines on lcd panel.
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Thank you, XXXXX XXXXX afraid of that but I had a feeling that that was the answer.