Originally Posted by Goran
make sure the card does not share 12v from the motherboard!
I am willing to bet its his PSU causing the problem because of what you just mentioned. This is why I prefer a strong single 12v rail PSU as it takes out all the guess work of a multiple rail PSU.
He is most likely overloading one of the rails hence when he games the card needs power and its not getting it=shut down.