With my SP4 plugged in to external power and in use, it gets warm and it adversely affects touchscreen sensitivity. The touch screen intermittently fails to respond to finger touch both with the on-screen keyboard and with icons on the task bar. It will respond to the pen. When I unplug the power and let it cool down, touch sensitivity will return to normal. Has anyone else had this issue? My SP4 is a few years old and I suppose it could be a hardware problem either with the battery or the cooling system.