Same problem with magenta chanel. Printer aquired in january, head fail after aprox 400 sq meters. About one cm of the magenta test print is doubled right in the midle. The print is afected only at high speeds reds. At normal speed is not a problem, at high res is not noticeable (also printing architectural drawings for students or pos posters). I'm still printing with the head, about 650 sq meters now, still doing his job no need to change it althou canon sent me a new one.
The ribbon cable to the head carriage has the light magenta data in the lowest thread, the part that glides in the profiles. On a friend's ipf9000 the ribbon cable has been replaced more than once due to wear there an related lm print issues.
Your problem could be related but not identical.
Ernst, op de lei getypt.