I saw similar issues discussed in some gaming forums where the PC just shut down when the GPU was under heavy load. The reason usually was that the GPU consumed more power than the power supply could work. Check for example. here or here . So is it worth investigating if your power supply is guilty.
Edit: Maybe SpeedFan can help you debug this - it can show both voltages and temperature sensors, which will also tell you if your computer is overheating (I never used the tool myself, and I'm also not connected to him, just found it online).
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