IPF 8400 misalignment

Hi!
I have a nice IPF 8400 printer that prints marvelous on glossy paper settings.
But when I use Matte paper (MBK ink) then I have an vertical missalignement (like a shadow on text and pictures. The MBK is slightly out of place.
The printer can be calibrated and head alignment without problem and this is only shown on MBK color.
On the head alignment sheet it is a line on the side of every square that are printed with MBK.
Yes I have changed the printerhead.
If anyone have som good input on this I will be very grateful.
Artist and printer from sweden
Bilbo