Is this an early 2008 model? They have a known GPU issue. You may have the NVIDIA issue. First, go to System Profiler > Hardware > Graphics/Displays and see if you have the GeForce 8600M GT GPU. If so, read about it here:
it was a free repair program, but it has expired. Still if you have that machine, I'd certainly go to an Apple Store and see if they will still cover it.