I have a Macbook Pro 15-inch, early 2011 model. I'm having some issues, it will suddenly hang and the screen is distorted with vertical lines on the screen. Noticed this usually happens when running more intensive apps (or many apps at the same time). Then when I hard reboot, I'll see the grey start up screen and that will also be covered with many vertical lines (and distortion on the apple logo). After power on and off a few times then it'll boot properly.

Also noticed some black powder coming out from inside.

Thanks for your reply. So that means it's a graphic card issue, right? Is it complicated to fix and will it be very expensive? Btw what's the black powder?