Hp design jet z2100 44"

Hi
I am recently getting some dots/lines on or around the printout
It does not happen everytime.
Could anyone tell me what causes the issue ? (picture attached)

 

Yep, My Z3100 does that too sometimes. The gloss black or matt black print head is stuffed and leaking. The ink builds up under the head and then proceeds to drip off and does exactly what yours is doing.

In fact, last week I replaced the belt and after I put the print heads back in mine was doing the same. Replacement print head solved the problem...