Responder a:iMac keeps randomly restartingI have the same computer as yours and the same issue for months. Last year I bought a new iMac to replace it (iMac Pro baseline) and since then I tried everything like you did to save the old one. I replaced the power supply as well. I detached original SSD and HD, once at time and both also, disconnected the motherboard, disconnected superdrive, tried to install a very old mac OS (Mavericks). I realize now it's been 5 years ago for you, but I only have read now. The strange thing is the computer is behaving like when there is a power loss, but after that is restarting itself and at login (when it works) doesn't say "Your computer shut down because a problem". When is on and powered for at least 30 minutes, that trouble doesn't happen anymore, and I tried the graphic test "Valley" for long time, with very intensive use of the GPU, so, GPU have to be ok. I'm giving up, though, don't know what to try anymore.