Definitely looks like you still have contamination in the paint.
Pretty sure I mentioned this before, but put a latex glove on a hand and use the clay with the other. When you've clayed the surface, rub your latexed hand over the paint to see if you can feel any more tiny "bumps" on the paint. If you can, clay more.
If it's rail dust, you may need Iron-X to get it off.
As far as doing it in the sun, remember that you don't have to finish up the entire job in one day. You can always divide up each panel and work on them separately during the evening or morning hours when the sun isn't beating directly down on the paint.
If that doesn't work, you could always look into using a water softening filter to remove the minerals from hard water that leave spots on the surface of the paint. There are many out there.
http://www.autopiaforums.com/forums/...zed-water.html