problem with volume display

Hi,

I have been using Display.RGBA map (value) with Linear3DSet (x,y,z) to
render a volume data ((x,y,z)->(value)), the higher the value, the lower the
transparency
with a pre-defined high color.
because the volume is pretty big so I make a divide the volume to be a set
of unit cube and interpolate their values by given samples.

My problem is when the cube partition is not dense, the volume display looks
like a set of "crossed-rectangles" for 3 sides(xy-plane, yz-plane, and
xz-plane) instead of
a solid cube. When I rotated the display, in some angle, there are some
"missing" faces in a unit cube so it looks like a set of flat rectangles.

Does anyone know how to fix it?
Thanks,

Tim