OK - So we know the iOS is clean and the digitizer it's self must be OK as you replaced it with the same reaction.
So, the next area to check is the connector its self could be damaged? It's also possible you have a bad contact, try cleaning the ribbon cable using a pencil eraser by wiping carefully with the grain of the contacts. Then use a damp stick sponge with isopropyl alcohol to clean the eraser residue off. As far as the connector its self you might try flushing it with some alcohol and using a dental vacuum to suck up the waste (or have it drip away from the rest of the internals).
If these don't improve anything you have a bad logic board (cold solder joint).