Black screen with backlight.

Got a Z3 from a customer with a nonworking screen. The backlight works, but there's no image. I changed the screen, it worked on first boot, then the same problem came back, no image, just backlight. I tried with yet another screen, still the same. Every screen works on first boot. Even if I change between two screens every time I boot it, it works every time, but if I boot two times on the same screen the problem occurs again. I tried changing the motherboard, but the problem is still the same. I'm really stuck, has anyone else ever experienced a similar problem?