It's possible you are having depth precision issues. If you tweak your near and far plane does the banding change size? If this is a problem you could possibly take a different approach and only change your blur width based on the first sample. I know the original Jimenez paper did a ddx and ddy calculation on the first sample of depth to figure out the slope.
Oh, also, just for testing, try sampling the alpha of your specular map on each blur sample and reject the color if the resulting sample is greater than 0. (I'm thinking my previous comment is wrong now, but I'll keep it just in case.)