Hi all. Maya 8.5. Precision 690. Windows XP Pro. 2 gigs RAM

OK, Parts of a particular complex poly aren't rendering as well as I would like.
These sections are partially lit so that the amount of light and shadow changes over the surface of a given section. However, rather than a visually smooth transition from slightly lit to barely illuminated, the surface will have crude gradations of shadow which manifest as waves or rings of tone. Where it appears it almost has the look of wood grain.

The whole scene doesn't seem have this problem, only certain sections of the geometry. Is there a way to fix this?

I've attached a picture to show the problem of a scene rendered in Mental ray.
I have various point lights which aren't casting shadows and one spot light which is casting Raytrace shadows.

I can fix this in Photoshop for a still image, but would like to know what causes this and how I can get more realistic renders in my scenes.

Thanks, T