It can be normal to find a few tiny dark spots in any cotton rag paper, due to some cotton husks getting in the pulp. I have seen this with many papers, including Epson media. What's not normal is if you find LOTS of spots, or large spots. I have actually found a perfectly intact fly imbedded in the surface of a rag paper that I was making a huge print on (44x60), which was obviously not cool, and made me question the cleanliness of the manufacturing or coating plant. With rag papers, you can sometimes gently remove the specks before printing, but unfortunately you can't do this with gloss papers without it being noticeable. Please let me know if you find many or large spots, and we will look into this further.
Regarding the gloss, you should get even gloss on Type 5 with the standard 30K GO at 1440dpi. Have you tried one coat of 30K at 2880dpi? What Harmon paper are you using? What is the recent average temp and humidity in your printing environment? I see you just bought this Piezography setup for your 3880 last month. Did you shake all inks before filling carts, including GO?
Please let me know.
Best regards and happy printing~ Dana