@stripes:

are you using depth-map shadows?

if yes, than increase bias (just a little bit, testrender, increase more if the stripes are still there)...

or just go with raytraceshadows ... look a lot better anyway

or use mental ray with final gather / hdri / global illumination, as it would fit best with such type of scene ... (with final gather you dont need any specific shadows any more, as the indirect illumination creates its own shadow areas ...)