Disable screen, or turn LS off.
Spray window cleaner onto screen.
Gently clean screen, and especially up to the edges, cleaning the rubber glue, and hardware buttons. Take 5 mins. Do a thorough job. Do this every week of hard use. Use paper towels.
Turn it on, and (as mentioned above) hold the AV button down to re calibrate the touch screen.
Little bits of foreign matter get around the sides of the screen. A good clean screen has fixed this every time so far.
Nate