robert, Ive had similar issues when making prints on platinotype (AKA 90# cover) and I believe Ray Bidegain mentioned in the past he had similar issues. I found when I opened another batch of paper the problem was no longer. So more than likely some sort of "bad" paper batch. Ive also experienced it from minor metal shavings that get left on the paper when I cut it with a paper cutter or exacto and dont wipe it off adequately.

hope that helps. Id give another batch of paper a try and see if that fixes it.