Hi all, long time lurker, first time poster! I've recently purchased a 2015 27" iMac with i7, 512GB SSD and M395x GFX card. The display has some strange artefacts behind the glass, almost like screen burn/ ghosting but not? I've tried all the recommended tests to alleviate this but have had no joy. Its most prominent on a dark grey background and not visible if I set my desktop background to white or black. It shows up almost like a zigzag rather than a hard straight line. Sorry, the phots aren't great and it looks worse to the naked eye. Can anyone suggest what the issue might be before I send it off for repair? Thanks in advance!