DJ Chris hit it on the dot.
XFX are horrible video card makers, and from what I've experienced, horrible motherboard manufacturers.
Your powersupply probably is too weak, because you bought it with the case, and thats why when you stess your system in those benchmarks and they look for some extra juice... your PSU can't give it and you crash.
Replace PSU first, and keep a close eye on that card.
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