I have had this issue today for the first time with many iPads. At first I was thinking it was pinched connector ribbons but after seeing this thread I no know it's something to do with the digitiser frame and iPad housing. I solved it by running 2mm 3m all round the bronze digitiser frame. This seems to replicate the foam padding/spacer that is in new iPads. It creates that seemingly essential gap required, much like on the iPad mini where the base of digitiaer needs to be taped over to stop ghosting