Digital BW, The Print

hi everybody

I'm getting lines on most of my prints that extend about a half inch
outside the print area.  These lines are all travelling in the
direction of the print head, and are caused by what looks like the
magenta head. In most cases it's just a few lines, somtimes its many
MANY lines, and every so often I get lucky and get a line free print.
Any ideas as to what's causing this and how I can fix it?

Thanks 
john