mate i have the same problem, i have ordered a new psu because of this.....its because the 12v rail on your psu may be outdated or may even not have one, so yeh thats what i got told, its coming tomorrow so i can tell you how it goes.
first off...if you had been using integrated graphics before,your have to disable it via BIOS
and set it to pci express
secondly ensure that you have updated your mainboard BIOS
lastly try running your machine with a better psu.