Monitor flashes green because of GPU

Hello, I recently built a computer for Christmas. Today I first booted it up and the fans were spinning and everything, but the monitor was flashing green. I was not able to see anything on the screen, just the monitor blinking green. The processor is an i5 3570k, the GPU is a Sapphire 7950. I tried taking the GPU out and it worked, but when I tried a different GPU the screen was still blinking green. The cable I was using to connect the monitor to the GPU was an HDMI cable. I tried it on both GPU's, so will I have to buy a DVI cable , install the drivers and then be able to use an HDMI cable? Thanks!