As far as i know, the granite node is good for values of x and y both greater than -2. That includes the scale factor, i.e. x/scale must be greater than -2. With a scale of 0.25 granite will mess up at x values lower than -0.5 (in poser units). So if your model contains polygons less than -0.5 granite will look bad on those; same goes for y (not relevant in your case). It should not depend on the light source, though, are you sure about that?It does this even without a light (in Poser 2010 at least). This is a bug in the granite node, not in the model.

A possible solution could be to export your model shifted to the right (in direction of positive x), of course this would mean that the wall's origin is no longer at the center of the wall, but it should be possible to shift it using Poser's joint editor (although i have not tried that).

BTW: the granite node is a 3d texture, so it is unaffected by a UV map. It models small veins in the z-direction. As a result the sides of the stone wall will look differently than the front.