Random vertical and horizontal lines flashing on my laptop screen

fine on the external monitor means the graphics card and driver are almost certainly ok, they're drawing the same picture for both. so it's the laptop panel or the cable going to it through the hinge. quick test: does it change when you slowly open and close the lid? if yes, it's the cable. at a year old it should still be under warranty, I'd get it booked in before it gets worse.