By the way, the symptoms of your problem regarding your PC instantly restarting after trying to run a game for a certain amount of time is almost certainly related to power draw. I had the exact same thing happen to me when I was overclocking my CPU. If I gave it too much or too little power my PC would do what yours is doing.
Tell me, did this only start happening once you got this graphics card? Does it only happen with this graphics card and does it happen without the drivers installed? If it is only with this card and only when you have the drivers installed you may have received a faulty card or a below-average GPU chip. I have three suggestions for what you can do listed from quickest to most time-consuming;
1) Apply more voltage to your card in Catalyst Control Center, about 5-10% on the slider should be enough. Don't worry about going too high, though; most overclocking utilities will only allow you to modify values within predefined "safe" ranges unless you change a setting which unlocks the sliders.
2) Underclock your graphics card on the same page in CCC, obviously not as ideal as the first.
3) RMA it and ask for a refund.
If I'm honest though I'd forget about #2 altogether and RMA it if the voltage boost doesn't solve the problem. Just remember to return the card to stock settings before you RMA it.
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