ArtRage will move the canvas if:
1/ The stylus right click button is pressed
2/ The right mouse button is pressed
3/ The spacebar is pressed
- while drawing. I normally recommend pressing each of these to make sure none of them are stuck. However, if that doesn't help, there is a cause which is a little more odd:
If you view shockwave flash content online, the flash plugin appears to set the status of the spacebar to down within the OS, then when you switch to ArtRage, ArtRage sees this and will move the canvas. Tapping the spacebar once in your browser or in ArtRage should correct this though.
I hope that helps!