Answer to: My Monitor starts flicering and it tells me to shut downIt looks like your iMac has the same problem as one a friend bought. If the capacitors show no damage than the most possible problem is the NVidia graphics chip. There were a few with bad soldering balls. This means that the iMac disconnects the graphics bus during running due to bad soldering. Some run 1min, some 5min, some 15min. I currently try to fix this problem by heating up carefully the area below the NVidia chip. Other people were sucessfull by putting the whole motherboard inside an oven at 250°C for 10min covering all of the board excluding the area with the NVidia chip with aluminium foile. At ebay Germany there is a company doing professional repair with a 20k$ ERSA soldering robot. But this is approx. 300$, a price you can get a working iMac 20". Apple had a exchange program but it ended a few years ago as far as I know. You can search for baking nvidia cards and you find a lot of solutions. Good luck.