Mac Pro won't boot into Apple Hardware test mode

I've been having some inexplicable crashes and graphic glitches, and it's been suggested it could be an issue with my graphics card.
Anyway, I've been trying to run Apple hardware test but it's just not happening. I have two drives in my mac pro, bay one has a LightScribe, bay two has the original super drive.
I can set the start up disk to the install disk when its in the original super drive and it boots fine into the install mode, but when I hold D (as it says to for the hardware test) but it always just boots as normal, as if I hadn't pressed anything at all.
Is there any other way to get into the hardware test mode?

Thanks,
Malcolm