the touch screen does not really "lose" calibration. the calibration is essentially a set of numbered coordinates that the controller uses to determine how close your finger is to those points it references (and thus it's position) this is why you must touch multiple portions around the screen when you calibrate. these values don't really change unless something is really wrong or something is interacting with the overlay.
firstly check your outlet and cord. it should be grounded. if some chuffer broke the ground off to plug it into a dollar store extension cord, or your outlet is not grounded or wired correctly... the whole machine can act erratically especially the touchscreen.
if you bring up the touchscreen test, press right in the middle to make an indicator appear and just watch for a bit and see where the cursor goes. see if it picks up any stray presses... place you finger in one spot and leave it there and it stays in one spot (or slightly jiggles under your finger) it's doing as it should. try pressing harder and lighter in different parts... if the indicator is off slightly (usually near edges and corners) re-calibrate. be sure to press and hold each spot for 2-3 seconds before you release.
if you notice it works fine in one half but is way off in the other (typically diagonally across the screen where the top and left are okay and the bottom and right are off or vice versa) you have a sensor problem. usually you can disassemble the monitor and clean all the crud out of the overlay. if it still is like this even after cleaning, a sensor is likely broken and it will need to be replaced... you can kind of emulate this by pressing your finger in the bottom right corner and using your other hand to press around the screen... you'll notice the marker is always about half way between the 2 points your fingers are at.
if you notice random presses are happening at the edge or corner, you may need to disassemble the monitor and check things out...
if you do disassemble the monitor, be sure the black tape around the edges is in good condition. if it is pushed off to the side and the glass overlay is touching the cabinet, it will cause all kinds of weirdness. find some TESSA tape or other high quality fabric tape to replace it. The reason for this tape is because the cabinet is covered internally with a metalized coating to provide EMF shielding and to make it emissions compliant. the overlay works by sensing a charge it applies to the surface going to ground when you touch it. the case essentially grounds out the charge and the controller thinks something is pressing on the screen. the touch overlay must be isolated from the case.
hopefully some of this information will lead you into what's going on.