Hi Christian,

It rather sounds like you may have a bum screen, but there could be other issues instead. Personally, I'd open it back up and reseat the connector; it could be just that simple. As long as you're in there, here are a few things you'll want to try.

First, get out a magnifying glass and a bright light and check both sides of the digitizer connector, on the motherboard and the cable, for any bent or broken pins. Bent pins can sometimes be straightened with some fine tip tweezers.

Assuming everything looks okay, get out some 90% or higher concentration isopropyl (rubbing) alcohol and clean both sides. Give them a moment to dry then reassemble and retest. If it's still being problematic and your old screen is functional enough, I'd suggest plugging it back in to see if the problem is still there on the old screen.

If not, that would indicate an issue with the replacement screen. Fortunately, iFixit's screens all have a lifetime warranty and you shouldn't have any trouble getting a replacement. From what I've seen they try their best to source quality parts, but no matter how careful you are, there can always be a few bad apples in the barrel.

Give these suggestions a try then come on back and let us know what you found