I have been wondering why the touchscreen is sometimes sensitive and sometimes not. If the PSU has a small high frequency ripple the touch screen is jumpy. If I swap to a good quality PSU it works like a dream.
The interesting thing is that a PSU that works perfectly with RPi 3 still does not work well with the touch screen.
The "official" white PSU for Raspberry has no problems with the touch screen.
Most ebay stuff makes the touch jumpy.