What exact brand and model PSU do you have? Not all manufacturers label their PSU's the same. Some may say 800w, but in reality they can only provide 400w of CONTINUOUS power, which is normally no name/ budget brands (for example http://www.hardwaresecrets.com/article/977
). When you see an 800wPSU for $40 and another brand for $100, you know the cheaper brand is skimping out on something.