Print problems with brother MFC-8860DN

Hi there, I'm experiencing print problems with our Brother MFC-8860DN, purchased in April 2008 and used in a home office. There are dark marks, "shadows", on the left hand edge of all printing and copying. They seem to follow a pattern, there are 3 bigger shadows spread equally down the page. I have cleaned the corona wire and the laser scanner window, printed off several blank copies, still the same. I then cleaned the drum, where there was an accumulation of toner on the LH side, by dabbing it with a cotton swab moistened with rubbing alcohol. When I tried printing again, the marks were still there, and the toner had re-accumulated in the same place on the drum. I have done this twice. Help!!