anyone know what causes spots on a model at render time? ....I'm not talking little fuzzy AO spots.. but big whacking ones in regular patterns!

(1 infinate light, IBL with a simple white to black gradient. 1 skydome (blue). Aircraft skin setup with a simple reflection node + texture and bump map. thats it - spots appear with both Raytraced and Mapped Shadows and with no shadows. Normals forward has no effect other than to raise render times. Spots appear with ground on and off. nothing else is in the scene. this is poser 7)