Screen Flickering but only when fitted, ok when not

I have an iPhone 4 that has had it's front assembly replaced. The replacement LCD flickers. So I temporarily plugged in another replacement screen and it displayed ok, so I went ahead and fitted this screen. HOWEVER, once the non-flickering screen was properly fitted into the phone and it was all put back together - IT WAS FLICKERING!! ARGH

The first screen I have now temporarily plugged into the phone and it is fine. So the problem only exists when the screen is properly fitted within the iPhone 4 frame.

Does anybody know what is causing this?

Thanks in advance!