ok, when I render with 3d motion blur, with a blur by frame value greater than .8, black spots show up on the moving model every so often, randomly. the only thing I've done that's gotten rid of it is lowered the blur by frame value and turned off motion blur altogether, but since neither of these are an option... anyway, I've also tried messing with depth map shadows and samples and whatnot, but nothing seems to work and I'm running out of ideas. any help would be much appreciated. thanks in advance

oh, I'm also rendering out in fields (I know normally you don't do both, but I've analyzed the original footage I'm comping into and there is motion blur there as well with the fields) and the spots occur in both fields but not at the same time