Going from the fact that you stated it only happens in the black, I would venture to say it is the ink combination on your black. Try sending a file with different stripes of black....
1. 100k
2. 20,20,20,100
3. 50,50,50,100
4. 100,100,100,100
Analize the results and work with your rip accordingly to reduce the amount of ink used to create your black.
jc