I have been using PMK for almost a year now. I have never had a problem like you are outlining. One thing bothers me. Why are you mixing the PMK then warming it. PMK oxidizes fairly quickly, especially in open trays. I would warm the water first, then pour in parts a & b. Now you have fresh PMK to pour into the tray.(posted 9034 days ago)I do not use distilled water except for photoflo and platinum printing. Try mixing the PMK with just filtered ta water. The more variables you can eliminate the faster you will isolate the problem.
I would also try another film.
In addition, tray development with PMK requires constant and substantial agitation. I rock the tray from right to left, then top to botton. This gets old, but results in no streaking. I would almost tend to believe that the spots are some form of streaking or inconsistent staining.
Please keep us posted on your progress.
Mike