Computer shuts down after some minutes

[Googolplex]

Forget about Amnesia. Your power supply may not be able to support the gpu even at 70% for example. It doesn't mean that it shuts down only when the graphics card reaches 100% power draw. Try the older 6600GT and then we will know if it's the power supply or not(it should be that though if not cpu TJMax).

CPU TJMax is at 100°C, so when Prime not reaches this temperature even with 100% at all cores, no game will reach TJMax.

I will first testing Furmark to stress test my GPU. And when my PC goes off, it's the PSU.