Also check the O-rings.  I have one that prints a perfect test print, then hit print and she stutters a lot.  In addition to the damper inline, there is also a damper in the print head.  I have been told the only way to replace the print head damper is to purchase a new head.  We have one machine we experimented with, and that solved our issue.  I guess after 7 or 8 years they just get tired.