I also have the exact same problem, I narrow it down to these:
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I also have the exact same problem, and gave up on it since it cost too much to replace, so I'm throwing this here in case it can be of some use to others, I narrow it down to these:
boot up OSX in safe mode = running fine
installed Windows on it '''with''' VGA driver (NVIDIA's) = instant shutdown
remove said VGA driver from Windows = running fine
so I guess it have something to do with VGA card, or the power supply can't handle VGA in full power
I also have the exact same problem, I narrow it down to these:
boot up OSX in safe mode = running fine
installed Windows on it '''with''' VGA driver = instant shutdown
remove VGA driver from Windows = running fine
so I guess it have something to do with VGA card, or the power supply can't handle VGA in full power