Screen is unresponsive to touch in a horizontal strip

My iPhone 4s has developed a dead spot that is unresponsive to touch input. The strip is about 1/4" in height and extends completely across the screen, at about 3/4 of the way up from the bottom of the screen. I have also had occasions where the entire screen became unresponsive when the phone was only mildly cold (like in my pocket on a 40 deg F day).

Thinking that the digitizing screen was at fault, I tore down the phone completely and replaced the display/digitizer assembly. This was partly successful, in that the screen and new digitizer work... but the dead strip remains. I am guessing that this is a problem on the logic board. I doubt that it is the connector; my cold temperature experience seems to point to the solder joints of the digitizing chip on the logic board. What say you?