I'd say a 550W Power Supply for that G/Card, even though it's a good one, would be something I'd try swapping for something slightly beefier.
//edit, I'm not saying it's the problem, but it's very close to the limit for a graphics card that's not the most power efficient.
//edit, I'm not saying it's the problem, but it's very close to the limit for a graphics card that's not the most power efficient.