In some configurations, which we've never adequately managed to exactly nail down, the high-precisions tablet coordinates we use are offset from the actual mouse coordinates.
To deal with this, we've put an option in the Edit/Preferences menu in ArtRage called 'Mouse mode'. This takes the X,Y coordinates from Windows cursor location, but still uses pressure and tilt information from the tablet.
See if that cures the problem.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"