Horizonal lines on screen: not LCD or motherboard

I replaced the LCD assembly on an AT&T iPhone 4. I didn't see them but a customer brought it back with some horizontal lines on the screen. I replaced the display and it still came up with the lines. I took out the motherboard and used it on the bench with a known good LCD assembly and there were no lines. I used it on the bench with the first LCD assembly and again: no lines! It seems that the lines only appear while in the actual phone body. Anybody have an idea what could be messing this up in the body? I don't see anything wrong in there... frustrating!