Tom,
There are 3 common things that can cause calibration issues:
1) "Dirty power", eg: low current/voltage, or a "flickering" plug. Make sure your connections are good.
2) Constant pressure on the screen can cause a temporary warp in the touchscreen layer. Such as storing it in a case with something pressing on the screen. If you calibrate the screen while it's still distorted, as it "settles" later you will need to calibrate it again. If you think you're seeing a touch screen issue because of this, turn the unit face-down on a table overnight.
3) If it's consistenly losing calibration and you've ruled out 1 & 2, your touch screen layer may need to be repaired. If you send it to our shop we can diagnose the issue, if it needs to be replaced we can do this for $119.
Thanks,
Walter