moving an image (background of layer)

For use in a menu I build a stylesheet which crops an image which
contains both the normal button-state and the mouseover-state.
On a mouseover the layer background-images 'moves', and another part
of the image will be visible (to be clear: the mouseover-image is
allready loaded). This all works fine, but a problems seems to occure
when the browser-cache is getting full. on a mouseover the movement of
the layerbackgroundimage is somewhat slow (first it turns white).

Is this a known problem? are there any sollutions/work-arounds
available?