'''In my experience one of two things could be the issue.'''
# You have a defective part.
# There are 4 foam pads that sit between the LCD and the back side of the digitizer. For some reason these pads will at times interfere with the touchscreen causing the digitizer to exhibit "phantom touch syndrome" as I like to call it :)
'''''These pads aren't needed''''' and I'm not entirely sure why they cause issues on some digitizers and not on others.
Let us know the outcome!