I had the same symptons a whilt ago, it was down to the gfx card overheating. Only the gfx card it seems has the power to TOTALLY hang your PC and also cause strange corrupt gfx at random. My gfx card fan had stopped working and it was overheating.
Give your Gfx card fan a bit of a clean, it may work for you. It did for me (5 month later still going)
Give your Gfx card fan a bit of a clean, it may work for you. It did for me (5 month later still going)