you'll notice when "jiggling" your GPU while it is installed that it still has some leeway for movement.the gpu is bolted with 2 screws.
have you gone through and re-seated all cables and components?I punched my table so hard my screen froze and mouse and keyboard lights went off.
most likely it's the hard drive, since the gpu is bolted with 2 screws.Hard drive going into shock protection mode or you jiggled the GPU in its slot would be my first guesses.
you'll notice when "jiggling" your GPU while it is installed that it still has some leeway for movement.the gpu is bolted with 2 screws.