my 2011 imac is going to have to go into apple, I'm guessing its the power supply although maybe not. basically what happens is it just restarts every minute or two. for a couple of months I had a good but weird solution which was if I managed to get steam open, civilization VI open and a save game open before it restarted then it would stay on for weeks. must be something to do with a sensor saying somethings wrong rather than a massive problem in that case as it must be that the computer is deciding to restart and is blocked by Civ VI. its not the internal hard drive as I bought a thunderbolt SSD. the graphics card was replaced by apple 2 years ago as was the screen. I'm leading to the power supply but techtools does flag up that the fan isn't working on the internal drive, so I normally unmount that. whats it likely to cost.