Well, the only two things that I can think of are:
a) something is adjusting the screen alignment settings
b) the hardware is going bad
In order to check the alignment settings, I would suggest monitoring them with CeRegEditor, an application that runs on your desktop/laptop Windows PC.
http://ceregeditor.mdsoft.pl/
You'll connect your PPC (Touch / Vogue) to your PC via the USB cable.
Once ActiveSync has recognized the PPC, you launch CeRegEditor and connect to the PPC.
Once you are connected, open the registry tree to:
HKEY_LOCAL_MACHINE\HARDWARE\DEVICEMAP\TOUCH
... and look at the following setting
"CalibrationData"=" Your Settings Will Be Here - Copy Them"
then use your PPC (while maintaining connection) and wait for the behavior you are experiencing ... i.e. the calibration go awry.
Once the calibration is off ... not correct or needing adjustment, refresh the tree in CeRegEditor and compare the settings that you see with the original ones.
If the settings have changed, then something has adjusted the calibration.
If the settings have -NOT- changed, then it would seem that something is not working right on the hardware end.
Best regards,
-boggsie