It's mental Ray playing up.
You could, select your object goto: Subdivision Surfaces >> Collapse Hierarchy>> and set the number of levels to 2 or 3.
Or convert your object to Poly's when it comes time to Render.
www.stevenegan-cgi.com
"Your weapons are no match for ours! People of Mars, surrender!"
"Um, this isn't Mars. This is Earth."
"Earth? Earth-with-nuclear-weapons Earth?"
"Yes."
[long pause] "Friend!!"