Cannot play games with Sapphire5970

Hello,
i recently purchased a Sapphire 5970 video card,but since i have installed it i can search the web watch movies but i am unable to use it for the main purpose of getting it and that is to play games.As soon as i try t load up a game it locks up my screen then reboot itself or the screen just stays frozen.I have tried reseating it in the pci-e slot,updated the driver for it but nothing seems to work i know its not the motherboard because i was using a Sapphire 5770 before in the same system.Any help will be appreciated.My PSU is silverstone st1000w,my mobo is asus crosshairIV