Lastest Drivers installed and the graphics cards are the Sapphire ATI Radeon HD 6950 TOXIC 2048MB GDDR5 PCI-Express Graphics Cards. There are two of them. The screen is the Dell UltraSharp U2711 27" Widescreen LCD Monitor - Midnight Grey. However the screen keeps flickering??? From centre to right. I am using the white cable. Any ideas as to why???