My first guess is a failing video card.
If you have another card, or internal video try using those and see if the problem still persists.
Also you could try resetting the PRAM and /or CUDA switch if your machine has one.
Finally after that I would start looking at the system RAM and swapping in known good modules if you have any and see if the problem is still there.
Also I would strip the machine down to the bare essentials needed to boot it and slowly add hardware back in 1 piece at a time until the problem reappears. It could be that everything just needs to be re-seated in their slots.
If you have another card, or internal video try using those and see if the problem still persists.
Also you could try resetting the PRAM and /or CUDA switch if your machine has one.
Finally after that I would start looking at the system RAM and swapping in known good modules if you have any and see if the problem is still there.
Also I would strip the machine down to the bare essentials needed to boot it and slowly add hardware back in 1 piece at a time until the problem reappears. It could be that everything just needs to be re-seated in their slots.