MacBook Pro Shutting Down Partially

[snckmrt]

I have a macbook pro 15.4" i7 and it shuts down when i use it. I mean, if I let it download something, everything goes perfect but when i watch a video or use a flash player included web game or preview my pictures or especially while i am changing desktops it shuts down. What is interesting at that point is the fans are still running, my iPhone is getting the electricity but it seems like the laptop is off. The keys doesn't work, the screen is black. I have to push the power button for at least 10 seconds to turn it really off. what is that problem. Can you help me with this. Regards...