Screen lights up, no picture...occasionally comes on?

Hi, I've been working on a Macbook that has some odd problems with the display. The Macbook boots fine but there is no video on the screen. Sometimes video would come on moving the screen around and pressing on the front of the topcase?

I figured it was the LVDS cable, so I replaced that. Same problem. Black screen, occasionally working. I replaced the display with a spare I have, same problem again!!

I also tried using an older Macbook screen assembly that I had laying around, and after doing this it would boot with a grey screen, and after a minute tearing and discoloration would cover the whole screen.

The Macbook works perfectly fine when connected to an external display. What's going on here??