Is the GPU in my MacBook Pro on its way out?

[Baldung99]

17” Macbook Pro (4,1) and I’m noticing some distortion on the built-in display but not the second display I’ve got connected. I’m a bit worried, but wouldn’t my GPU failing be obvious on both screens?

 

[Amethyst1]

I've had a MacBookPro5,1 with a similar but much worse issue on the internal display, and the issue was the LVDS cable that connects the internal display to the logicboard. Does the distortion change when you open the lid further or close it slowly?