I have a complete Vue scene that I opened in Maya 8.0 and attempted to render with mental ray. I get large artifacts and black areas in the render that do not show up when it is rendered in Vue (crap render engine by the way). I have turned scanline off and set shadow calculations to segment as recommended in the manual for Maya-Xstream integration and rendering in MR. Still I get the following:

If I render the same scene again, I get the black artifacts in totally different areas!!
Does anyone know what the issue may be? Much appreciated in advance.
Josh