unstable or intermittent LCD

I have an issue that's stumping me. I had a routine iPhone 4S screen replacement with strange results. I am the owner/operator of an iPhone repair shop and I've replaced hundreds of iPhone 4/S screens.

1. As I always do, I tested the new screen by connecting it to the phone before disassembling. The screen was blank.

2. I reconnected the old screen with a known working LCD and that screen was also blank.

3. I connected the old screen to my iPhone 4S tester circuit board using the eTech Parts extension cable and the LCD worked.

4. I connected the new screen to the tester circuit board and the LCD worked

5. I checked under the microscope and everything on the phone's circuit board was in tact. There were no broken resistors or connectors.

I thought, ok, something made this phone not happy about me unplugging the screen while it was on. I'll do the repair and then a reset since I see no damage on the circuit board and both screens work.

6. I did the repair and a reset and the new LCD was blank.

7. I plugged in the old screen and the LCD was blank.

8. I checked both screens in my tester circuit board and both LCDs worked immediately

9. I tried several reset and changing screen combinations. Finally the old LCD started working. I plugged in the new screen and the LCD worked.

Great! Done! Nope.

10. I tried rebooting the phone with the new screen just to check that everything was stable and the LCD was blank.

11. Repeated step 9 and got the new screen working again. Rebooted and the screen was blank.

I still see all resistors, connectors, etc. in tact and not damaged when I look under the microscope. There's no damage to the cable as I can get the new screen working just fine in my tester circuit board. Any ideas or suggestions? I'm a little stumped.

Thanks.