Macbook Pro random shutdown
It sounds like your motherboard has suffered heat damage.  The most likely culprit is doing something 3D graphics intensive, because the graphics chip can get a lot hotter than the CPU but the fan speed is controlled by the CPU temperature.  Another possible cause of overheating is running Windows in BootCamp.

I've had that happen on a couple of different Macbook Pros (earlier models) and once it reaches this state you need a new motherboard.  In my case, they were still under AppleCare so it was free.  Since then I've always made sure to crank up the fan speeds before doing anything 3D intensive, and I installed fan controllers on the Windows side.