I've actually had something that is a little like your promlem. Only mine doesn't happen every time, and I'm not sure if my latest update of my video card has removed the problem or not, I haven't seen it for a while - but then again it doesn't happen every time.

What I get is WHITE pixels on a render. Apparently scattered at random, I haven't checked if they correspond to the bucket size,and it's not as neatly spaced as your dots by far.

My white pixels seems to be transparent IIRC, I remembertrying to get rid of them by pasting a darker background behind the render in Photoshop. But othersise I've been blending them out. It is doable because there isn't all that many of them. But enough to be a PITA. Interestingly enough, moving the camera ever so slightly and re-rendering has solved my problem every time so far.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.