Hi, A few months back there were very faint vertical lines on my late 2009 27" iMac. They were only particularly noticeable when editing photos or looking at the screen very closely. A few months back I opened up the iMac, removed lots of dust which I think was causing over-heating and did a fresh install of OS X. The lines seemed to occur less frequently and I hadn't had any issue since, until today. Today my iMac suddenly froze whilst I was repairing disk permissions. The screen was covered in bizarre white squares. The mouse moved, but otherwise the entire system was unresponsive, including force-quit not working. This was the first time this type of thing happened. After attempting several hard reboots, black vertical lines appeared on the startup screen and the system would not boot (it got stuck on a dull grey-white screen after the Apple logo had disappeared and the lines remained). I attempted PRAM and SMC resets and booting in safe mode but none of these worked. I then attempted to enter recovery mode, which didn't happen, but the system instead booted and there were no issues with the graphics. After 10–20 minutes of using the iMac as normal, the same happened again, except that the squares contained lines of multiple colours. I attempted PRAM and SMC resets, which didn't help. I then repaired the disk and disk permissions in recovery mode. Still it wouldn't boot. I then managed to boot in safe mode but this time the lines (now more like thick bands of vertical grey lines, evenly spaced across the screen) persisted. After reaching a certain point at startup, the bands gained an extra blue line in the centre, 1px thick. Having restarted again, the bands are now completely 'noisy' and multicoloured. EDIT: I just noticed that by simply selecting an area on the desktop, these bands disappeared. I was able to remove them all by dragging over them. I then restarted and again the system froze as usual after the Apple logo disappeared. It seems that I can only boot in safe mode. I have read that this could be a CPU issue but is most likely a GPU issue. I have also read that (at least in laptops) the CPU and GPU are built into the logic board and I know that iMacs use some laptop components – is the GPU built into the logic board in this iMac model? If so, should I risk assuming that the GPU is the cause of failure and replace the logic board? Are there any other possible causes of this problem? I want to avoid having to have this repaired professionally. Is there any soldering involved or is this something I could do myself? I no longer live in the country where I purchased it (Sweden) and there are no Apple stores (only resellers) where I live now (Finland). It's obviously not covered by a warranty. Thanks for any advice!