I have an hp1055cm plotter

Question

I have an hp1055cm plotter that is leaving streaks on the page i.e. it makes a clean first pass and on the second pass (as the print progresses)it is like the head does not reach the paper and leaves the second pass light. I have changed the printheads and the problem persists. When the plotter is cold, it will complete 3/4 of the print job successfully and then the streaking begins. As the plotter is used more the streaking occurs shortly after the print job begins. Any suggestions?