QTR-Quadtone RIP

Hello al, 

While printing my 21 step gray scale in BO mode, I
noticed that I had microbanding. I made nozzle clean and check,
realignement... But without any change. 
This is a SP 1400, with Eb1400
kit. This microbanding happens only on the black head (Eboni 100%). 

I
have the feeling that when I start printing the microbanding is not
there, and after a few prints it's getting worse. 

I would be very
grateful if anyone could give me a trick.... 

Thx 

Sylvain. 

[Non-text portions of this message have been removed]