I would first try swapping data cables with an other head, if its still happening on the black head, try a new head.
Don't forget to unplug the machine wen swapping cables, I also press the green button a couple of times just to get rid of residual electricity.
If this fails, then your pretty much screwed, its either the main board or the slider board witch is more expensive to replace then the printer it self.
Sorry !
P.S. you might want to go here for more help : Mimaki
