[X4U] Mac Mini refresh "artifacts"

[nk]

One of the apps I use is Painter..version 7, on a Mac Mini 1.42ghz

often, when I zoom in on a drawing to work a tiny area, there are what 
I'd call artifacts
from each successive zoom level, so if I  hit Cmd-+ 5 or 6 times, then 
there'd be 5 or 6 "repeats" of that area of the drawing on the screen.

Same if I use the hand tool to move the drawing around.

zooming in one level and back again clears it, but I wonder why I'm 
getting this at all...

the mini "knows" what's there. if, after zooming in or scootching with 
the hand tool, why doesn't it draw it correctly?

thanks for any insight


nk