Display problems after replacing screen

I recently replaced the LCD/digitizer in my 1st gen iPhone. The repair went smoothly, and I reassembled the case. When I turned it on, the screen flashed random colored lines.

The backlight was lighting up, but a clear image would not display. I proceeded to take the case off the back, and try and power it up disassembled, and the screen worked fine. As soon as I put the case on, however, the gibberish resumes.

I have checked and rechecked the display ribbon cable and its connector, and both seem to be fine. Also, if I bump the phone hard from the side while the phone is apart and the screen working fine, it will lose the signal and go back to the random lines.

I tried restoring the software, and it seems there is no problem. The computer recognizes and syncs with the phone, and the touch screen works fine when the image is clear.

Is this a problem with the ribbon cable? Anyone have any experience or ideas?