I've seen something very similar when converting contour-map data into greyscale bitmaps and then into a terrain in Vue. The terrain ends up with lots of little steps, and they vary a bit depending on the bitmap format I use. I get rid of these using a quick 'diffusive erode' in terrain editor, but when you are using it directly to drive bumps, it may not be so easy. Are you exporting as a greyscale image, or in a high-colour mode? Saving your image in 16 million or more colours may help, but I don't know. We may be seeing different things, I don't know, but remember, there are only 256 levels in a greyscale picture.