I have no solution to the problem, but I think everyone is missing kawecki's point. Refraction bends light, the light leaves a medium at a different angle from that at which it entered the medium. The effect should be constant. If you put a stick into a bathtub, it will appear to have a kink at the point where it enters the water, but only at that point, the part before it enters the water will appear straight, and the part in the water will appear straight (providing the water is still)! Real refraction does not create sudden discrete jumps, except where an object crosses from one medium to another.

Is the refractor a single or double sided surface? If double sided, try using flattened cube in its place, with a little space between the two opposing sides of the cube, or try using displacement to separate the two sides of a square. Poser seems to have problems with double sided surfaces with no separation between the two sides. This is not guarenteed to work, but it's probably worth a try.