I had the same problem with my MacBook x3100 for ages, I took it back to John Lewis in the UK where I purchased it from. Using their 2 Year guarantee they replaced the Logic board as it was toasted.
I have never really used the mac in anger, my personal opinion is that snow leopard caused the graphics problem on my mac book. I doubt that apple will fix the issue as with us existing laptop owners not contributing new revenue to apple they will just continue to develop iPhone/iPads/iStuff and not really fix anything for people using old technology.
I am a bit disappointed with apple having previously owned Powerbooks/iBooks and G3's with no problems at all
Thanks