Digital BW, The Print

I've been using Black Only on my 2100 for a couple of years with good
results.
Now I seem to be getting very faint lines in the direction that the
print head lays down the ink. It's most noticeable in plain medium
gray areas. The other thing is, it seems to come and go. Sometimes
it's quite pronounced, others, it's barely discernible,
Does anyone have a solution?
thanks
Yaakov Asher Sinclair