Hi im trying ot overclock my Sapphire HD5670,but every time i try to go higher than 800mhz on my gpu and 1020mhz on vram i get malfunction. When i start a game the screen turns black and i need to restart the computer,Ive tried changing the bios of the vga but same thing happens. Im sure its not heat because i have installed a Zalman VF1000 and it runs on 28C.Please help i don't know what else to do to overclock my graphics card.
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Try upping the voltage.
You can try increasing the voltage as romanrp said, but make sure not to raise them to much and keep on eye on temps. Excessive voltage and heat can damage GPUs.