White dots on 3000cn printing.

I'm getting white dots on the printer output from a Dell 3000cn printer.  As a test, today I printed four pages, each with a large colored rectangle in black, red, blue and yellow colors.  The red and the blue rectangles had the dots; the black and the yellow ones did not.  These are faint white dots 1/2 to 2 inches apart in a somewhat regular pattern.  The black cartridge is at the 25% level and the others are between 60 and 80%.

How can this be fixed?  Thanks for any help.

James