The metal contact might be it...
I noticed if you touch the silver metal contact with your finger as you have the back cover off, the performance of the touchscreen increases. This is what I am talking about.
Here is what I believe to be the problem, correct me if I am wrong. That metal contact is meant to ground the phone and it is not working properly or maybe since most of us use rubber or plastic cases, we block that from happening. Since the static has no where to go it just builds up on the phone, and in turn messing up our touch commands. Capacitive touch screens work because our body is a conductor. The screen is coated with a transparent conductor, so it makes sense that static would mess things up. I forgot who but someone said earlier that they charged the phone on a metallic surface and the problem went away, charged it on wood and the problem came back. I study electrical engineering, but I am still a student and I could be wrong but its a start.....right